i feel like these charts are much harder than charts in game of the same difficulty level... is there a way i can compare the difficulty scales or something

The difficulty in custom charts is pretty much just vibes based, especially compared to base game. There's a site called TootTally which hosts community leaderboards for custom charts which also has an automatic difficulty calculator but that one's even worse in that regard. Base game charts barely crack a 5 and Eine Kleine (Champ Mix) is a 3.7. For higher level players, anything the base game considers a 10 might as well be free.
It's kinda annoying that the difficulty is capped at 10. Compared to base game, a lot of charts would be in the 20s or even 30s range but the game breaks if you do that.

For this collection the difficulty is mostly representative of how hard the chart is compared to others within this collection. I might revise it though I'm not sure.

oh i see good to know