Fellas when you are in single player? Hit start, go to Online then Play GTA Online. You will see options like invite only, crew mode, solo mode etc etc. You do not have to go to the public lobby where guys can bother you unless you want too. Yes everything is exactly the same in solo or invite only modes. Trust me I've done that past two days. You can still hold up stores, still do missions. Also you can if you want do deathmatch, race, whatever by pulling up your phone or going to the symbol in the area.
Second if you want to stack up on money? Do missions for Gerald. Simeon missions are easy. You can put those on HARD so you rack up on XP to level up quicker however his payouts are anywhere from $1,000 to $3000. If you're solo you can do these missions by yourself. Gerald missions you can do by yourself but its quicker with a teammate or two. At any rate here is what you do. Call Gerald or Simeon and they will give you a mission. If the mission pays out well keep REPLAYING the mission over and over and over and over. The missions you replay they spawn you AT the mission do no driving all the way across the map to do it.
I've done this mission called No Hay Bronco from Gerald. Its $9,000 every single time. I even did this solo and once you figure out the pattern you can fly by it. I've done this mission straight at least 20 times. Why else am I posting at 4am.
I have close to $300,000. Level 23. If you want money do this mission or Violent Duct from Gerald. Simeon I cant think of the name but his missions are basically REPO missions. Go to destination steal car, lose cops (wanted level) then drive to his shop. He gives good XP. I had a mission with him where I was getting 1400 XP each repeat. Levelled up quick.
2 options you dudes might want to explore.