I think you're assuming there's only one kind of bottom surgery?
There are two different types, meta (metoidioplasty) and phallo (phalloplasty). Both have their benefits and shortcomings. In the mainstream trans community, the shortcomings are overstated and the benefits are understated.
Meta is smaller. A lot smaller. Micropenis size, usually-though it depends on the amount of 'clitoral' growth T gives you (meta is constructed from the 'clitoris'). I've heard anecdotally of a transguy who had 4 inches when hard, but this is extremely rare (read: is not going to happen to you). Think more around 1-2 inches.
At the average 1-2 inches size, it's a bit of a crapshoot whether meta can penetrate or not. However, it has full erotic sensation. I'm not sure whether the head would be the most sensitive-that's not really something I've read about before.
Metas don't require any kind of implant to get hard. For a meta, you can choose whether or not to get urethal lengthening. Obviously, you can't STP without it (yeah, yeah, I know, I know, STP devices...I'm talking without one of those things, here). There's a somewhat high risk of complications, but bladder control tends to be fine-complications are usually more related to blockage and the like, I think, though I'm not 100% sure. You can also get a meta without a colpectomy (a colpectomy is what's also referred to as a 'vaginectomy').
Phallo is the one that has the size of an average penis. It's generally thought to not look as much like a CAMAB dick, but that's generally kinda inaccurate nowadays. (I saw an absolutely AMAZING result once, but the place I saw it escapes me.) It always requires a colpectomy and urethal lengthening.
You don't need an erectile implant-you can penetrate through other methods, and the implant has a risk of migration, which is about as bad as it sounds-but all the ways of having sex without an implant sound like a complete headache, and I imagine you'd need it for anal penetration. It wouldn't make you permanently erect (though some older methods did).
Phallos can't get hard on their own.
Unless your doctor is a hack job, you can penetrate anally with a phallo. (Unfortunately, there are hack jobs out there.)
The base of a phallo is actually more sensitive, as the 'clitoris' tends to be buried around that area. Phallos absolutely do have erotic sensation, though-it just takes time to come back. You know how it takes time for nipple sensation to return after top surgery? Same thing here.