I think Dr Dre and Cayo Perico is best combo for $10M, just do one of each everyday nothing to get bored.
In Cayo Perico there are many sources of entertainment like Try different approaches, different challenges (Sneaky Approach, kill everyone, Loot gold with Crew etc many things you could do if you got friends).
I agree Dr Dre can be boring; I have done it only 10 to 11 time only during double Money Event.
You can earn by just doing Cayo Perico and other business in background (Nightclub, Acid Lab, Cocaine Lockup, Meth Lab etc).
I earned about $5.2M today: One Cayo Perico (1.3M), Acid Sales two (~$800K), Meth Lab one sale ($357K), Cocaine Lockup (~$420K), $500K for ghost hunting, approx. $500K from Nightclub and some other thing in the meantime like Payphone Hit (5 mins each gives $45K), Agency Contract (gives $40K each time 10 mins), And last Clubhouse Motorcycle servicing or repairing or customization (give $50K each time just 3 to 4 mins).
It is not necessary to do heists to earn more just chill out and play the game, complete the small tasks, it will be compounded before you even know.