I could be mistaken about this, but I think quest rewards scale with either your charm factor, charm skill, or both. I almost always get more reward for turning a slave in than I paid for them in the slave market if I buy one. I usually just turn in one of my free ones though. I generally start my main character off with 5 charm factor, partially for that reason. I also find mages to be stronger early on. They take more damage but they also deal more damage and have better healing.
I agree with your character build, my main char tends to be Dragonkin, while my assistant is a Drow/Dark Elf... but I also concentrate on sex, since slaves gain more exp and stat Bonus from intercourse, and it's a great way to increase loyality... also mages have more elemental attacks early on (and if the user is an elf you even get AoE skills) which grants them increaseded damage
You can get 2 free points to magic, charm, sex, and tame affinity that way and 1 to everything else except growth. Don't think there's any way to boost growth so I always start with a 5 on that for my first slave. So with a bit of min maxing you can get both yourself and your first couple of slaves up to great or excellent in all but a couple of stats in the first week or 2. I don't do an affinity booster unless I can get it to excellent without spending any resources.
I make a point in creating high rated human slaves since they have a natural +10% gathering Bonus (+10% from being a slave), humans come with terrible (1) or poor (2) growth rate at best, so I breed them with my assistant to get 2 human futa slaves (I have 0% on males in the game... only enough space for 1 bull in my estate) with a Great (5) growth rate (since I upgrade my assistant in the process those kids also tend to be on Good (4) with the remaining stats).
Afterwards both human slaves become breeding sows (children inherit the mothers stats and pregnancy is accelerated by 30%) and keep impregnating each other so I constatnly gain 2 new Slaves.
Later in game apply the same for Wolf girls (Hunting), Nereid (Fishing), Dryad (Farming) and Dwarf (smithing/Tailorring)... Daisy is likely the only one you need to turn into a whore since Gold by whoring is hardly worth the skills and efforts it requires
I hadn't paid much attention to that but I usually pick irresistable (boosts consent) so it never takes more than 2 tries before a new girl will do group sex. It may well be more of a pain with a different starting sex bonus.
That perk is more useless in late game though since consent can be raised with several skills, I normally have a trait that increases learn rate or arrousal of the partner