Yevamos 24B, Tosfos there, Yevamos 47a-b, 76A.
Rambam hilchos isurei biah, located in Sefer Kedusha), perek 13, halachas 14,15,16,17. ( hebrewbooks.org rambam browser makes this convenient)
Tur and shulchan Aruch Yoreh deah siman 268, especially the end. I didn’t see the Shulchan Aruch yet, but in the tur it’s sif 4 and 12. I have a Tur set at home but not a shulchan Aruch set.
The gemara quite clearly states that they didn’t accept geirim in the days of Dovid and Shlomo because they geirim had ulterior motives. The Gemara does say that there were 150k geirim then. The Rambam says that they were converted by batei din of hedyotos. The Hagohos Mordechai (Yevamos siman 110)says that if we know they have ulterior motives, we do not accept them. the Rambam does say that if they were accepted by others and had mila and tevila, bedieved (as in, not lechatchila), they are full geirim and we watch them to see if they accepted the mitzvos. The Tur is quite clear ( by siman 4) that kabalas ol mitzvos is me’akev, even though a simple reading of the Rambam doesn’t say it. Kabalas ol mitzvos is me’aekev even if it wasn’t in front of 3, kol shekein if it was neevr done at all. That is why we can go lemafrea. However, even the Rambam is mode that we don’t accept geirim lechatchila for ulterior motives. I only put in an hour into this, and didn’t have time to read through the shulchan aruch nosei keilim and later poskim yet, but it does seem quite clear that 1. kabalas ol mitzvos is me’akev, at least according to the Tur, 2. There are those who say the Beis Yosef agrees with the Tur, and the only heter is if they didn’t inform him of the mitzvos, assuming he meant to accept all the mitzvos, not that he didn’t fully accept all the mitzvos, and 3. According to all opinions, we do not lechatchila accept geirim converting for ulterior motives. Geirim who accept all the mitzvos for ulterior motives are geirim bedeieved, but lechatchila we do not accept them. Geirim who do not accept mitzvos are not geirim (based on tur 4 who says even kabbalos ol mitzvos in front of less than 3 or at night is bad). So to lechatchila make geirim out of goyim for no good reason, and who don’t want to accept mitzvos, is AGAINST normative halacha. The only question is bedieved, if a beis din of hedyotos (a good name as nay for abeis din that goes against the lechatchila of normative halacha) if the Rambam is in fact arguing on the tur and holds that bedieved kabaalas ol mitzvos is not me’akev. However, the beis yosef holds like the Tur and has a different girsa in that part of the Rambam. This wasn’t some chumra made by Rav Eliashiv .