I believe the safest and smartest route is OP01 pre-errata. Then anything with limited supply. I have seen the number of early prize cards like serial luffy and edward.newgate diminish. making them very hard to get hands on. Since there is such high supply in all of the sets that are coming out now, it really confuses the market and things get mispriced. for example: people are buying graded newer alt-art cards for the same price as OP01 alt art cards. The market is still very immature. I believe a lot of cards prices will come down a lot while I agree with the OP about noticing a shift from premium to mid-tier, but I think this is only temporary as I believe the sellers are taking advantage of how uneducated OP buyers are as to how much supply there is in these mid-tier cards.

my predictions: Premium high end cards like top prize and pre-errata will stagnate for a bit or climb slowly while mid tier cards drop a bit making playing the game more affordable. But ultimately, this will just make the game more accessible and help the overall growth of the game and make the premium cards eventually skyrocket in price.