Gaining a Low Investment High Damage Unarmed Strike

[noob]

Wait you are playing 3.5/pf1 or 3.5/pf2?
Also since when can you multiclass between systems?
Can I play gurps/vampire/exalted then spend hours trying to guess how things works?
Anyway as you can understand there is a lot of feat support for unarmed strike but unarmed strike shines best when you can apply a lot of dice increasing damage modifiers(else you are better off using another weapon).
Some of the most basic ones are feats(and even if sudden kick does not increase damage it still is a good feat for unarmed strike shenanigans) but you can also get size increases(enlarge person for example) and virtual size increase(the weapon is not actually bigger but it hurts as if bigger for some reason) please note that pf prevents stacking increases from size so get ones that are as big as possible.

As base dice increases are best when you can get a lot(and thus is investment heavy) the above posters were right in telling you to optimise it as if it was any other melee attack.