In my country at least (UK), it's not yet possible for an individual to pay to have a booster and so the only way of getting another vaccine shot is for free via the NHS. However, this is only available if you are sufficiently old (I believe over 50) or in a "vulnerable group", so teachers, someone working in healthcare, or someone who has a pre-existing medical condition that makes them more likely to have more severe effects. So, far from making it more open to all, this suggestion massively limits the potential attendee pool.
Of course, the other suggestions like not attending if you have any symptoms or having negative test results, seem like good steps in the right direction.
Masking is another issue - maybe it'd be OK for a 1 day event, but personally, I wouldn't attend a 3 day event if I had to wear a mask the whole time, because they are uncomfortable for prolonged periods. I was super vigilant during and after COVID and always wore a mask when I would encounter people, but because I didn't enjoy wearing a mask, I decided it was better to largely stay at home where I didn't need to interact with people instead. I'm happy to agree that the preference to not wear a mask is selfish, but it's still an interesting question on how many people would choose to not attend because they don't want 3 days of discomfort compared to how many vulnerable people would choose to attend if everyone was wearing masks. My gut feeling is that people who are that vulnerable would still rather stay away and avoid crowds because there would still be a non-zero risk, but equally I can see that some people might be glad for an opportunity to finally do something in a larger group that's safer than normal larger groups.