Welcome!
Hmm. Interesting questions. As for what it's like being in a relationship... much like it was being in a relationship with her before, except with a year or so of transition hassles and trauma in between. (I've never been explicitly dating a woman - though I'm bisexual and there were some friends with benefits situations when I was younger - so I'm not sure how to compare, but I'd imagine it's a lot like dating a cis woman except with the transition drama.) But we'd been together a long time by then, so it's a little different, I think; we had to readjust to each other, but the coping skills we'd learned from growing up together (we met as teenagers) helped. Anyway, I don't know if that's all that helpful, but for your specific situation I'd stress that this will not be a totally different person; there's likely to be a lot of changes, big and small, but the basic personality will probably stay more or less the same. I also don't know of very many people who found that their sexual orientation changed so much on HRT that they abandoned a long-term partner; in fact, just as there are heterosexual cis women who stay with a trans wife because they can make a special exception, so there are trans women who aren't willing to torpedo a relationship because HRT shifted their primary preferences. That was the thing I was most afraid of, and in the end my wife was bisexual at the start and is bi now, though she had a somewhat increased attraction to men - say from 90-10 in favor of women before to 75-25 now.
As for what can offend your partner, that is up to them! ("Ladyboy" is considered offensive by some trans people, but if your own partner doesn't mind being called it, that trumps strangers' opinions.) Communication is key here, just in general.
We didn't want kids, so I can't help much there. As for the bedroom activities, well, you have been attracted to women, so you know that an organic erection isn't necessary to have fun.

It's possible that HRT will cause problems there (it's possible that it won't), but there are even trans women who wear strap-ons despite having a penis. There are a lot of workarounds.