I've been reached out to by a recruiter to interview for a fully remote Senior Dev position at a seed stage startup, with around 50 employees per LinkedIn.
Ive only ever worked for larger companies before, what sorts of questions should I make sure to ask during the interview process to make sure there's no hidden problems/wrong expectations? What expectations should I have going into the interview?
50 people is pretty large for a seed-stage startup, interesting. Seed stage is generally 25 people or less.
... what sorts of questions should I make sure to ask during the interview process to make sure there's no hidden problems/wrong expectations?
I recommend this thread for a very similar question to yours that talks about how to figure out if a startup has real legs.
When it comes to startup interviews, expect them to be far scrappier and intimate as the people interviewing you are almost certainly going to be with your coworkers if you were to join. Live coding is very common for startups (e.g. I had people build a small Android app when they interviewed for my team at Course Hero), and take the behavioral round extremely seriously.
Passion for the mission is also critical for a seed stage startup, so do a lot of research on their product vision beforehand (try actually using the product if you can).
Lastly, since you seem to be around the senior level, here's a good discussion around good questions to ask in general when considering a new team.
One thing to understand is the process through which they get things done. 50 people is large enough where not everyone knows all the projects going on, but it's likely too small/new to have a bunch of mature processes.
Also, get a sense of the financials. Understand how they actually make money.
Seconding Alex's statement that 50 large for a seed stage. I'd go as far as to say that it's excessive in almost all cases. My first concern before figuring out if it's even a good fit would be primarily financial:
With no other context I would expect a 50 person seed stage startup to lay people off due to excessive hiring in this climate. Your questioning should be thorough enough to fully rule that out. If the company is actually solid they shouldn't mind you asking these questions, and should understand your concerns. If they're cagey and take offense to these kinds of questions I would take it as indication that they know their decision making has been poor and aren't being honest with you or possibly even themselves.
If you get past the above angle of questioning I recommend figuring out a specific project that seems to be a good fit between the company needs and your own skills and personal goals. One of the main benefits of a startup is expanded scope and impact, but there's still a constraint on business needs. I touch on this in depth in the Taro session I did with Alex over the summer https://www.jointaro.com/lesson/8hT0g5peO8fwDZmvvIFZ/session-6-maximizing-swe-growth-ownership-and-specialization-at-startups/