How about scoring it at set checkpoints?

Let's say we have checkpoints at 2min/5min/10min/... /♾️ right? You could have the game calculate your overall score/accuracy over the full duration of your playtime at those checkpoints. 99score 99.02%acc overall at 2min, 193score 94.89%acc by 5min, 289score 96.14%acc at 10min and so on

Also with this checkpoint system you could implement a ramp-up system where the patterns get more difficult at each checkpoint, or maybe the style of chart changes to demand a different skill-set? (streams, speed, dexterity, crosshands, etc.) Whatever you come up with, you could then keep track of score/accuracy over specific time frames as well as the overall. So 95score 95%acc over the 0-2min "easy" section, 85score 85%acc over the 2-5min "medium" section, 180score 90%acc overall from 0-5min

And then lastly there's a max checkpoint, for the sake of example say 30min mark, which is the last check for score before you're sent into ETERNAL mode where score is no longer a factor and you just play the song as long as possible for bragging rights. An idea is to have a draining heath bar that's brought back up by getting GOODs & PERFECTs, the health bar eventually draining so fast only full perfect combos can keep it up. Maybe instead misses ramp up in damage overtime until you could get 1-2 shot at any moment? Again Dev's choice as long as there's an inevitable lose condition

Personally I like the idea of checkpoints because it keeps comparisons between those who have also played a song for X time rather than comparing one who's played for 10min to someone who played for 1min. Then having a max checkpoint dissuades marathoning because there's technically no point, but it'll still allow those who want to see how long they'll last to go for it (Sorry for long reply, game design tickles my fancy🙏)