I don't know how to code, but I was able to make a simple mod that causes Tarquin to follow you throughout the game.
He's still a trader with all the same functionality, and there are a few issues (he won't fight in the Driftwood arena, he will still just pray for you in the final battle, he detects crimes even though I tried to turn this off, and he can't be resurrected) but otherwise it works from what I can tell. He'll still wander off to Stonegarden, and this mod does not add him to Fort Joy, but as soon as you get to the cemetery gates, he'll follow you for the rest of the game. I gave Tarquin Arcane Stitch, Rain, and Blood Rain for a little more utility. I tried to do the exact same thing with Almira, but she won't teleport with you between areas, maybe due to some scripting relating to her questline.
I've really been enjoying playing through with Tarquin hanging around the whole time. So I was wondering, would it be possible for anyone to figure out how to give all this functionality to other characters like Almira, fix the bugs, or to implement features like making Tarquin level with the party or become controllable directly like Delorus/Saheila? I think full-scale mods that turn major NPCs into full followers would be a really cool idea.