You may wish to rethink your Driftwood route then. I typically wait until level 14 or 15 to do Wreckers Cave. Larian has made the game open world. You are completely capable of getting into areas well of the level of your party. However, there can be good exploration XP in these areas as long as you avoid conflicts. The following quests can be completed at level 9 : Counting your Chickens, Treated Like Cattle, The Ugly Little Bird. The XP gained from doing these can allow you to change up your route a bit allowing you to skip things like Wrecker's Cave or just the Mordus fight until later in the act if you so choose.
As for increasing your source, you only have 6 choices. However you are choosing not to do two of them, Mordus and Hanag (Lizard sorcerer). This means you need to choose 2 from the following 4 : Riker (Elf running the graveyard, likely level 13-15 to complete his quest for him to give you source), Saheila (Elf captured by Roost, the lone wolf, level 14 for this), Jahan (Deomologist, level 15 to kill the advocate) and Almira (succubus, level 14-15 to kill the voidwoken causing her issues.)
I play on tactician and rarely, if ever, use source skills in Driftwood. You can wait until late in Driftwood to increase your source capacity. It just means you won't be busting out heavy hitting spells until the Nameless Isle. Nothing wrong with that.
The point is, you have options. There is no need to follow the theoretical prescribed route in act 2. In fact, changing up your route, can give you a fresh play experience. You have options to gaining more source. However, a couple of them come extremely late in act 2. Choices are plenty and do not feel forced into anything. I hope this gives you some ideas of how to change up the way you are doing things in Driftwood.