The g'mara on Rosh Hashana 14b says (Soncinco translation):
But is it right to adopt the harder rule from both sides? Has it not been taught: ‘As a general principle, the halachah follows Beth Hillel. If one prefers, however, to adopt the rule of Beth Shammai, he may do so, and if he desires to adopt the rule of Beth Hillel he may do so. One, however, who adopts the more lenient rulings of both Beth Shammai and Beth Hillel1 is a bad man, while to one who adopts the more stringent rulings of both Beth Shammai and Beth Hillel may be applied the verse, But the fool walketh in darkness. No; either one must follow Beth Shammai both where they are more severe and more lenient or Beth Hillel both where they are more severe and more lenient’?2
You ask: "Are we allowed to be Machmir on ourselves in Krias Shema and read like the opinion of Beis Shammai?" The implication here is that a necessary precondition is that you follow Beit Shammai for everything.
Whether, having adopted the customs of Beit Shammai, you must still not do this one because of (historical) danger is not clear to me. But presumably Beit Shammai did in the past (even with danger) because it was their tradition, so if you want to wholly align yourself with Beit Shammai you might be able to make the argument that you can do this. But if your goal is chidur mitzvah, not to affirm Shammai, consider instead the approach suggested in this answer.
1 An editorial note here says "on the same subject" but I don't know why they say that as we don't pick and choose our traditions (e.g. you can't be Sefardi for kitniyot but Ashnenazi for something else).
2 Cast as a question here because this is part of a discussion, but what follows makes it clear that this policy is not in question.