I'm very pro remote. But there are times where in person collaboration is faster (usually at the start of a new/large initiative where lots of stuff is undefined). It's also a lot easier to build high trust relationships in person, because so much of communication is actually body language and you lose that over video. Maybe you can pitch a middle way, where the team is mostly remote except for 1-2 weeks per quarter where they all come in to focus on some collaboration-heavy work? And then emphasize the impact to the business if N people from the team quit over RTO. If you can't find a middle way, I wouldn't necessarily give up. I'm an IC and never been a manager, but I'd never convey a command or decision from upper management to my team that I couldn't personally own and stand behind. So I would ask her for the justification behind the RTO mandate in terms that you can own and sell to the team. (The answer might be "no, I'm not going to justify it" and then you need to decide how much you like your job.)

Keep in mind she might have received some shitty KPI like "number of engineers badging in every week." So the middle way might be hybrid once a week, if that helps her numbers move. She may not be a nice person but if you can understand what her motivation is or what metric she's trying to move, you can figure out how to serve her and the team.