Many minyanim nowadays daven mincha very close to sunset and then daven maariv right after. To me it just feels like a way to get maariv out of the way, and also it makes the 2nd Shemoneh Esrei (of maariv) less meaningful considering you just said one. Does anyone talk about doing this and whether it’s permissible or not ideal etc?
Mishna Brura רלג:ט says that those who regularly says Mincha after plag hamincha, such as the minyanim you mention, should not to say Mariv until after tzeis hacochavim (the earliest time for that would be 13 1/2 minutes after sunset). Not for the reasons you suggest, which I think are more an area of personal preference, but because the time between sunset and tzeis hacochavim is questionably nightime, and Mariv should be said when it is certainly nightime.
However, I do remember seeing someplace which ruled more leniently (but I can't manage to find that source right now).
I did ask my Rabbi about this once, and he said that the custom is to be lenient and therefore he wouldn't tell anyone not to do it, but since I was asking (presumably, he said, because I was looking to do what's lechatchila), he would tell me it is better to say Mariv in a minyan that waits until after tzeis hacochavim.
EDIT: @DoubleAA Granted the minhag during the time of the Rishonim was to daven very close to Tzeis 1, as proven from the R"i, but the OP said davening maariv after Shkia. I merely meant to prove that even before shkia was not an issue. I did not say the sun was high in the sky at all.
Actually, during the times of the Rishonim they davened even earlier!
People davened Minchah and then immediately davened Maariv while it was still daytime!
EDIT: It seems I have been misinterpreted. By the words "still daytime" I mean under 10 minutes before the beginning of Bein Hashmashos."
Why did they do so? Because it was too bothersome to come back to shul and daven at night! (Rashba on Berachos 2a).
If you will then ask "How did they daven Maariv so early?" That is not a question because the Gemara explicitly says that you can daven Maariv after plag Haminchah (Berachos 27a).
If you will then ask "How did they fulfill Krias Shema so early?"
There are MANY Rishonim who discuss this. There are three main opinions:
Daven normally and say Shema over at a later time. (Rash"i, Berachos 2a D"H Ad Sof, R"i Ib'N Gaitz [quoted by Rashb"a ibid.] and Rav Amram Gaon [quoted by Rashb"a ibid.])
Daven only Shmoneh Esrei with a minyan, and say over Shma with Birkhos Krias Shema later. (Rav Hai Gaon quoted by Rabbeinu Yonah Brachos 2a. [Maybe also Rabbeinu Yonah himself. [I am unsure about Rabbeinu Yonah, I am away from my Gemara right now.])
Daven Normally and that is it, you already fulfilled Krias Shema then in shul because the zman of Krias Shema begins earlier than Tzais HaKochavim. Rabbeinu Tam, Ri (both in Tosafos Brachos 2a D"H Mai'aimasai).
The Shulchan Aruch paskens like Rashi with a slight modification (Orach Chaim 235:1).
So to answer your question, there is nothing wrong with davening Maariv right after Sh'kia, since people davened earlier for hundreds of years...
Regarding ethics, Rabbeinu Yonah says to say K'rias Shema immediately after Tzais HaKochavim and the Shulchan Aruch (Orach Chaim 235:2) paskens like him! Also, the Gemara in Berachos 29b says it is a mitzvah to daven Minchah as close as possible to Sh'kia, because of the pasuk (Tehillim 72:5) Let them fear you with the sun, which Rashi says refers to Krias Shema.
THe only kasha you might have is regarding the fact that it seems that you are treating it as a load, rather than a privilege, which the Gemara Berachos ibid. does mention. That most likely refers to davening very quickly, hanging out near the door to be the first to leave etc. not specifically davening at these times, which the Gemara and Rishonim explicitly state are the best!
For further reading see the following:
Gemara Berachos 2a, BErachos 27a.
Rashi Berachos 2a D"H Ad Sof.
Tosafos Berachos 2a D"H ma'aimasai
Rabbeinu Yonah Berachos 2a.
Rashb"a BErachos 2a.
Tur Orach Chaim Siman 235.
Shulchan Aruch 235.
Shagas Aryeh Siman 3.
1 I don't know how to do footnotes, but by tzeis I mean 13.5 minutes after the start of Bein HaShmashos, not the chumra of calling the seventy-two minutes between shkia of the gra and tzeis of rabbeinu tam bein hashmashos...