i did the same tanking one season (9) to train my youth stars (4/5) and i became champion of the league

and could build the 73000 stadion

but i never used the same formation only counter formations that season and i did have a few older players around 24/25 age 4/5 stars

in my team

My suggestion try a mix with 4/5 stars and a few older players around 24/25 and look at the stats how they perform before buying (24/25)

Maybe its help