Strategy guide: Upgreade 1 automatic and tree as much as you can, while leaving the remaining 3 auto guys on level 1 (just upgrade once): the guy on the left of the tree gathers money dependent only on tree level unless very well upgraded and sicne every upgrade ups the cost of every other upgrade no point in investing here.
Ok, why is growth practically zero after you hit 20-something years? I understand, after 60 or 70 years its hard to learn new things, but still possible. IMO you should improve stranght and dex when young and int and will as you age.
Infinite money trick - do misson 1, kill the boss tantacles, and sell all units, leave just brusers, he will keep spawning units worth 24$ each, just wait long enough and you can buy every upgrade. When bored - just buy ranged unit in middle to kill the boss.