What Homebrew is possible?

[Caribet]

I'm only just looking at HLO. (Been using PCGen for many years).
I see thta "most" of the Paizo material for Starfinder is covered as paid-for extensions (with some glaring gaps).
I also see a number of posts asking if specific aspects can be customised.
Can I instead as the more general question:

What, if anything, can be Homebrewed?

• Class - too complex, not expected
• Race - SF races arn't that complex, and often just recycle standard features
• Archetypes - like classes, I can live without
• Theme - also typically simple, with a boost here, and option there
• Background - just text?
• Equipment - level, cost, weight, description, (possible game effect)
• Weapons - specifically have damage type & size, specials, critical effect (standard list)
• Armour - weight, cost, level, KAC/EAC, expansion slots, special properties (from a list)
• Ammunition - bulk, cost, level, damage + type, specials (from std list)
• Spells - could be complicated, but many are no more than flavour text + casting time, range, duration, effect dice

Is there any "macro language" or formula system?

Or are we restricted wholly to what Lone Wolf have crafted and sold?

(Homebrew Race and Theme are top of my list as the campaign is starting, followed by equipment & spells.)