3000 sites, is not a small job. So, as they are not your domains i would tell him he will need to invest, else they are worthless as the days of domain name arbitrage are largely over. Now you have to buy, add value and then sell.
Ai is not that difficult to use, you just need to play about with it to get the prompts right for your needs. Then you could perhaps combine it with a text spinner, have a standard site design template and probably best hosting on cloudflare.
Read some of the journeys about it on here to get more ideas, but that is the general direction i would go for site setup. Then obviously you need to rank them.
If he wont invest in adding value to his domains, walk away. I would charge him $120,000+ to set it up for 3000 domains, it is only $40 each and more to rank them. And if he did not commit to all 3000 with 50% upfront i would again walk away.
If he agrees to invest and you dont want/know how to sort it, set up a jv on here, i am sure someone would help for that sort of money.