First off, congratulations on getting an 8/8 with only one character slot! Seriously! I don’t think anyone has said that yet, and it needs to be said. My first 8/8 was made on the back of a dozen 0/8s that died collecting pots for it. Quite the accomplishment for your first one to be self-sustained, essentially.
As far as where to go from here, I must agree with the others. At this point, you kind of have to decide how much you like the game and think you still want to play. If you are like me, I found that I really do honestly enjoy the game, and playing on different characters is a huge part of that! For me, spending $25 on the game seemed totally fine, especially because I already had like 100 hours of playtime by then. When I compare that to other games that cost $60 and only have like 50 hours of playtime if I am being generous, it seemed like a great deal. (Let alone if you compare to movie tickets, which are like $8+ for <3 hours of entertainment…)
Some people are totally against microtransactions, and I understand that, but I prefer to think of it as the price of the game itself. Despite how a lot of people complain about the game, I still have a blast playing it! Just don’t get too bogged down in trying to amass wealth… There are only so many Hall runs you can do before going a bit crazy. Running around doing some events and godlands dungeons can be a really chill, fun time, even if the in-game rewards aren’t too great.
Enough of my rambling.
TL;DR: buying a few character slots will greatly improve your experience of the game, and could easily mean the difference of a few hundred more hours of playtime. It is worth it!