Hi there tenderqueer,
From what you're describing, it sounds like this person didn't have your consent for everything they were doing, which means that yes, it is sexual assault. Sexual assault is any kind of sexual activity without the other person's consent. Tribbing is definitely a sexual activity, and is one that many people would consider to be sex.
I can't tell you how much of the ickiness you're feeling is down to these specific instances and how much is about other assaults. I don't think it's a question that really CAN be answered - these traumas have a tendency to compound one another, building up layers of hurt that can't be meaningfully separated from one another. I'm so sorry you've had multiple experiences of assault, and I'm glad you have support from a therapist for it. I know it's a slow process but I promise you it does get better.
Can you move past this history and have a healthy relationship?
things often still feel a bit icky and heavy for me, like I feel kind of repulsed by them a lot of the time.
This sentence very much tells me that you aren't there yet, and pushing through those kinds of feelings generally isn't a good way to heal.
I wouldn't advocate being involved with someone who has hurt and assaulted you in the past. I do understand that a lot of time has elapsed, and if this person has confronted what they did, taken responsibility for it and done the work to re-educate themselves then I'm sure that they can be a safe and loving partner to someone else. It doesn't sound like a place you're feeling safe right now though, and working through that trauma whilst it's constantly being activated by your relationship with this person sounds doubly hard. What does your therapist say about this relationship?