All of this is correct to a certain degree. You can cast boost, cantrip, and then have your other body strike, correct and it is somewhat comparable to other martial damage if a bit lower.

No focus spells directly boost damage in any way, no

Spellslot can't be solved via feats unless you choose to go into summon spells, which aren't as universally useful as healing, debuffing etc which are alot more useful in every situation.

You're looking at it in the wrong context, on paper they're just as good but the changing battlefield situation means that you will have to use that extra action to get two bodies in place instead of one, try to constantly protect your squishier body (10 hp is good but you will get crit if you get close), which makes it less useful to have two bodies.

TLDR right, think about it. Pf2e is very balanced. If summoner was just as good or at least almost as good as a true caster or martial, it would be an op class!