There are some basic concepts about making brachos (blessings) which should be understood as a background for any answer to the OP's issue. These concepts are vital to the proper attitude one needs in order to make brachos at all; and will greatly help in clearing up doubts about Hilchos Brachos in general.
1) It is forbidden to enjoy (benefit) from this world, without blessing G-d first.
(See the beginning of Talmud Brachos Ch.6 This is to say that the blessing is a proper "Thank You" to avoid stealing from Hashem.)
2) We are very careful not to utter a blessing which is not needed, or in vain. The Halachic term for this is either "brachah sheh'aino tz'richa" or "brachah le'vatalah"
(See Shulchan Aruch O.C. 216:4 where it is equated with taking Hashem's name in vain.)
3) For everything that you could have enjoyed in this world during your life, that you avoided, or didn't enjoy, you will be asked in Heaven (after you pass on) to justify why you did not enjoy it.
(See Talmud Yerushalmi Kiddushin end of Chapter 4)
4) All blessings are Rabbinical Law except for "Birkas HaMazon" (blessing after a meal, when one feels full; which is D'oraisa (Torah Law). And, all Rabbinical Law therefore follows the general rule of: "Cases of doubt are treated leniently." (safek drabbanan l'kula) So, we therefore follow: "Cases of doubt in blessings are resolved leniently." (safek brachos l'kula).
(See Shulchan Aruch O.C. 209:3)
Now that we know this, we can continue to address the OP:
The OP writes: "I just had, baruch Hashem, a delicious meal of pizza followed by dessert pizza at this place. But now I wonder if I will ever have it again, because I have no idea what, if any bracha to make on the dessert."
The first thing we must say is that avoiding the dessert pizza is absolutely not an option for Jewish life. Should a G-d fearing Jew really risk judgment in the next world (Olam HaBa'ah) by not eating this fabulously yummy dessert?
As we see from the Yerushalmi mentioned above, skipping dessert pizza may be a sin! But, as we see from the Gemara and Poskim, eating it without a proper blesing first, may also be a sin! And, making the wrong blessing is certainly a problem!
Therefore, the best option in the first place: (based on the OP and the case of having already eaten some regular pizza as a meal and then having more dessert pizza at the same sitting) would be to make no new blessing on the dessert pizza and simply include it in the original Hamotzi and the same Birkas HaMazon after the meal.
This mostly matches the OP's option #3: "Nothing; we rely on the hamotzi made at the beginning of the meal to cover the meal as well as this dessert which was not part of it."
We do not avoid eating it, because we should enjoy it!
We definitely do not make a second washing and a second Hamotzi, because that would be a blessing in vain (and your hands are still ritually clean, so why wash again?).
When we hear in Halachic terms that a dessert is not "part of the meal" it does not mean to say that it is entirely not part of the meal at all; rather it is not part of the main meal, but it certainly is included in the overall sitting. You are eating the dessert within the same meal. Also, if you think that the dessert pizza is defined as true bread so as to need a "second Hamotzi" then it really is included in the meal under the original Hamotzi as true bread and not a dessert, despite it being sweet and consumed at the end of the meal. So, two Hamotzi blessings for bread would always make the 2nd blessing unneeded and therefore in vain. IOW, the dessert pizza is either bread or mezonos. There is no third category called "Hamotzi mezonos".
The only question is do we make a mezonos on it instead since it is a dessert and maybe it is "pas habah b'kisnin" instead of being true bread?
If it is really bread, and you make an extra mezonos, that would be a blessing in vain. If it is really mezonos dessert, and you skip the mezonos blessing, then you didn't say the extra blessing you were supposed to say!
The answer to this final issue is :
1) Ask your Rabbi what your custom is concerning this type of pizza and its brachah inside a meal as dessert. Make sure to ask without assuming a "psak" so as to avoid vows and avoid contracting this Rabbi's answer as binding to his honor as a Rav. Make it into informal guidance/learning. Then follow your custom. Once you rely upon a valid custom, you have no problem with doubt because the doubt has been legally and legitimately decided. If later you learn more and feel more comfortable following a new custom or another opinion, you can do that. If its a difficult situation and you need to rely upon a different opinion you may do so as well. Remain informed, yet flexible.
2) If you have no custom and cannot verify with a Rav, or the information and opinions you get seem to conflict etc. Then this is the reliable path:
Shulchan Aruch 167:9 explains that if one has a safek about saying a brachah (for instance Hamotzi) then one does not make a brachah. The Mishnah Berurah explains that 1) It is assumed that the person continues eating without the brachah 2) The blessing is only Rabbinic and so when in doubt we are lenient, and 3) Making the doubtful blessing anyway just in case, is not an option because its a blessing that isn't needed. (might be taking G-d's name in vain for nothing)
Finally, we can add that the loss of the mezonos blessing in our case, cannot be considered as eating without a blessing so as to be a thief who did not properly thank G-d.
This is because you have said HaMotzi already at the beginning! Your original Hamotzi on the first pizza certainly shows you thanked G-d for the meal! If so, what possible reason do you have to risk saying mezonos?
"Nuances" :
1,2) Even if you had dessert in mind or bought it, you still follow your custom about dessert blessings. Those customs were established knowing that people would be aware of dessert in advance.
3) The strategy of putting bread on top of the Dessert pizza would not cause a new Hamotzi obligation in the middle of a meal. Such a "new" Hamotzi would be an unneeded or wasted brachah.
4) If eating the dessert pizza after bentching is over or after a non-bread meal, one would be starting fresh. This then becomes a standard question for a Rav on how to treat this pizza? 1) The best way to avoid all issues is to wash and say Hamotzi on 2 ounces of regular bread first, and then eat as much of the dessert pizza as you like, then say Birkas Hamazon. 2) From the description of this pizza, it sounds like a form of real Hamotzi bread. Certainly if one treated it as bread, (even without using regular bread first) they have opinions on which to rely. 3) If one finds a reason to consider it pas ha'bah b' kisnin, then it can be mezonos. With that approach, it is better to eat a snack sized amount. Criteria for such consideration and what is called a snack, has been provided in part by the link in the OP which illuminates further study.
5) Dough baked alone might be considered Hamotzi even if other things are added later because of the Halachic rule that once something becomes bread, it never loses its status as bread.
6) The more one eats, the more it has a chance of being treated as bread because of the Halachic rule of "koveah seudah"; you treated it as a meal.
7) Whenever one treats something as true bread, then you wash netilas yadayim before eating it. Birkas Hamazon covers everything you ate before.
In all of these cases and more, a Jew is supposed to establish a care-free easy going way to sincerely make a blessing with kavanah (intention). It should be a moment of true conversation and thanks between the Jew and their Creator. Without the obligation for blessing, there is a lack in enjoyment. Without a full pleasure and enjoyment, there is a lack of obligation for blessing.
I hope this helps. :)
