I am a Librarian and have run several different D&D games there as my job. I strongly recommend the following things: 1) run the current version of the game, OR run something like OSR and advertise that heavily. 2) expect a varying group each week. My rule was that I kept all character sheets and repeat players had first dibs on who they played. 3) try to run the game as SHORT episodes. Wrap each one up at the end of the session. Recap any crucial info at the start of the next session. 4) Have a really clear endpoint session. Every 6-8 weeks. At that point, start over with new characters and a new overall plot. This helps with closure for people who are unable to commit to a LONG campaign.
I was never able to run sessions that were longer than 3 hours, partly because it was my job, but also people didn’t want to commit that much time. I made sure games were very “G” rated, but I also had a tapout card for each player. If something was off limits for them, they could throw the card on the table and I would steer the game off that topic. Never actually had one used, but we all liked the idea.
My library owns and circs a full set of Wotc 5e books, and a few others. It is amazing how steadily they circulate. I think we have some PF2 as well.