Sizing up and a speculum question?

Looking for recommendations on sizing up... At the moment I can get toys between ~3-3.5" in diameter in my ass. What do folks typically go for when they move up a notch (or two) in toy sizes? I'd hate to go too small and conquer it quickly and then need a new one and have to spend more right away, but I don't want to make the mistake of getting something impossible either. I have a large unflared chance, but it feels like it's too long, also have a big bad wolf inflatable from primal hardwere which I love, just wish the knot would hold it in place better. Been looking at the Flint, Crackers, and Pierce, but open to other knotted toys as well.

Also curious if folks out there play with speculums at all? If so, recommendation there would be appreciated. I play by myself so would need something I can operate on my own and have it stay in.

Thanks for the advice!