Nothing medical or surgical is "required" in order to be trans. I was gender dysphoric before I medically & surgically transitioned; meaning I've been "trans" [I don't use the trans label socially/in daily life, just using it here for clarity's sake] all my life.
Requiring HRT/Surgery is no longer an inherently Transmed/"Truscum" belief. If an individual is Transmed and also happens to believe all trans people must go on HRT & undergo surgery in order to be "truly trans", then that's just that individual's [who happens to also be Transmed] personal belief. Transmedicalism is ONLY the belief that one must experience gender dysphoria/incongruence in order to be trans.
Your wording in the title is a bit vague on what exactly you're referring to (are you meaning "required to pass" or "required to be trans"?).
"Now, this is specially bottom surgery for MtF I’m talking about. Top surgery for FtM I can absolutely see as a requirement. But I don’t see the requirement to have bottom surgery as a “trans thing”."
Top surgery is often necessary if someone [who started HRT post-puberty] wants to pass as male, yes, but there are rare cases where it isn't necessary (as in: being extremely flat and/or using puberty blockers before natal puberty).
"But after a couple years on HRT, and the right treatment, MtF.. ‘default’.. genitalia can definitely be more feminine than it is masculine."
While that may be true, that doesn't mean the genitalia itself is now female, and therefore, the existence of said genitalia—regardless of masculine or feminine in appearance—can still cause gender dysphoric feelings in trans women. It's perfectly fine if the feminizing effects are enough for you to now be comfortable with your genitalia, but this is not the case for all trans ladies. Gender dysphoria can manifest itself in different ways; what works to allieviate it for one person may not be enough for another.
"Personally I think it comes down to how you and your partner treat it."
Again, this can be helpful for some people, but not all.
"And this all doesn’t take into account the (while lower chance of it happening) horrendous possible outcomes of surgery, and even if it goes well, all the new needs of that orifice. All in all, I just don’t see why bottom surgery for a trans female would really be anything but a personal decision for that person."
Unsavory results can come from any surgical procedure.
A relative of mine just had the most painful foot surgery one can get. Should they have not gone through with a surgery they needed because there was a high chance of being in immense pain afterwards? No, because you gotta do what you gotta do. If you need something, you need it—no matter how difficult, how complicated, how painful, etc., it may be to get the thing you need.
Some trans people need bottom surgery. Just because I, you, and others happen to not need it doesn't mean no trans person needs it; many do.
"I would argue that proper voice training is far far more required to be a trans woman than bottom surgery is."
Like I said earlier, the only absolute requirement [in the eyes of Transmedicalism, at least] to be trans is to experience gender dysphoria.
I think you're unintentionally equating the word "requirement" with "things that will help a trans person pass better as their true gender" throughout this post.