Clubhouses are just for Biker content. Biker businesses are slower to make money from, too. In a lot of cases, if you Buy Supplies, you actually lose money on a lot of the businesses, especially without all the upgrades or selling local.
Cocaine and fake cash, selling far, are two of the better secondary ways to make money if you have friends to help (or get into a solo lobby/get lucky to get left alone). Buy supplies if you sell a full load, you make pretty big profit for almost no work. But there's a lot of waiting in between. If you're doing single deliveries, Export cars or crates are better, and also give far far more RP, too. If you're doing full loads, the return on time and money is great, but again it's not really a primarily 'work' to do solo.... you'll buy supplies and wait around for 2-3 hours, and then spend 10-30 minutes trying to delivery either 3 vehicles (or 1 vehicle if your lucky). You can constantly do single (1 vehicle) deliveries too (every 45m or so) but then the profits are worse than the CEO stuff.
I like the Biker content, and I think I'm up to 130 deliveries or so, but I mostly do it for the style/roleplay/etc... I'm pretty damn poor from it lol.
If you just want make money, Heists are probably best if you got a pre-made set of friends, so buy that high-end apartment to do Heists with them.
But if you're solo probably invest in the CEO stuff first, and do Exports I guess, and either Crates or VIP work (Sightseer first and then Headhunter if you need another). So, CEO office (Arcadius probably best cuz it's central and get a warehouse nearby so it's fast to to fly back to), and then a vehicle warehouse that has a big space to land with the helicopter). Then Buzzard and Cargobob, I guess. After that... maybe Bunker for upgrades? Or get your Heist stuff so you have that as an option.