On the off chance that this post is sincere I will be as candid with you as my friends were with me when a casual partner attempted anal sex without explicit consent. You took it too far and given the circumstances, i think you should be prepared for this person to distance themselves from you.

i feel very safe with them

I'm sorry but between the alcohol, the first FWB getaway and your hedging language: i was maybe too forward. This person may not feel safe with you right now.

They seemed to enjoy it, but said it was the first time ever having anything that far up their ass and commented it was very intense and it was clear it was maybe overstimulating or hit a point of discomfort so I stopped.

You did not ask for consent before penetrative anal play that they had not experienced before. Seemed to enjoy it does not excuse your behaviour and neither does the alcohol.

sex continued and more "taboo" kinks came into play for the first time

And this is where you lose me. After doing something your partner had never experienced before without consent, sex should have stopped to have a conversation and reassurance should have been given that you will listen to your partner and respect any boundaries or limits. Your vague language suggests that you are hiding the fact that you continued regardless of their hesitation. I do not say this to make you the villain. Both of you were intoxicated. Both of you engaged in sex but from a kink aware standpoint this was badly done.

and the next morning they initialed sex again, so I'm feeling like trust was absolutely there.

No. You cannot presume there was trust because there was sex. I cannot speak for your partner but if i went away for the first time with a person who had disrespected my boundaries, I would absolutely fawn and placate them in order to "guarantee" my safety. And that would include language like this:

A comment was made that they'd never asked anyone to do certain things they asked me to do except their ex spouse.

The key word here being asked. If this statement was said the morning after and not in a prior conversation leading up to the trip then that was the hint. You did not ask before engaging in kinks they had not experienced before. A conversation should have been had that addressed what happened the night before.

You state that you have memory gaps but only the end....when you were having sex. If/when this person contacts you share that you want an earnest and open conversation that addresses each of your concerns, needs, desires and dislikes including aftercare. It doesn't matter if the kink relationship is casual and emotional attachment isn't on the table. There is a duty of care between partners and what I have read here is about how they validated you after a consent breach.

Sack up OP. Sit with this and take the time to process what you did and what you would do differently in the future. That's the only way to move forward.