So realistically, I can ask my boss for a video meeting, or I can ask them when they'd expect to be in the office for an in-person meeting. The former feels a little inappropriate in how informal it is.
It is not informal, it's just... "different" than what the majority of us are accustomed to.
Remote working is no longer some futuristic concept that may happen one day. It is what is happening right now, and it is quickly becoming the "norm".
So do not think a video call, or even a phone call, is informal. It is perfectly acceptable given your current situation.
My advice for what you should do is to prepare an email detailing your resignation - include all the information you would in a written letter. Strictly speaking, this is all you actually need to do - fire this email off to the relevant people (i.e. HR if you have one).
HoweverThen, if you feel that you have a good working relationship withdrop your boss and prefer to give them the heads up (which is what you seem to want), then just drop them a chat message or email and ask them if they are free for a quick call (or schedule one in if they are busy). Phone or video, whichever you prefer. Then you can tell them your intentions before you send the official notice via email.