Any tips on having a startup with a mix of in-person and remote work?

I believe that arrangement gets called hybrid work, or work from home (WFH). If you need or require people to work from an office together even one day a week that means they must live reasonably close to the office. That in turn limits your candidate pool, perhaps severely depending on the location.

If you can manage people working remotely part of the time, why not all of the time? You get access to a much larger, possibly global, pool of candidates.

Nothing to do with startups, really. I suppose a startup might benefit more from in-person collaboration, but that seems more a function of the team members and communication than physical presence.

You might find that more senior people and those with specialized skills, who don't struggle to find a job, will strongly prefer remote only. I have only worked remotely for about 15 years and I know quite a few other senior people who only work remotely.

You’re going to have to pay them a lot and have a lot of office perks to attract them vs your competition on the job market. Or accept lower caliber of candidates.