I'm worried about fees going too high when Bitcoin gets mass adopted. (I don't need to explain, 1 block every 10 minutes)
If Bitcoin fees go to $100 or more per transactions, then it's not viable for low and middle class, for mundane tasks like transferring some to someone, DCA a monthly low amount,etc
Notice that you're contradicting yourself. If fees make it non-viable for middle class, how can it reach mass adoption?
There's no solution other than using an L2, unfortunately. And I get it, the most developed one right now, LN, is sometimes ugly and expensive to use, but there is more development on L2 that make use of lightning, like this one:
https://arkdev.info/. Divided possession of an UTXO is one way to improve scaling drastically.