we run in a similar gutter (agree with all your stances there).
what kind of climate are you in? i'm in central CA and as an example, am am abandoning fabric pots because it's too hot & arid, they dry out unevenly, & then i over-water. so next time they'll just line some big plastic grow bags.
the humidity here is so low that VPD is only available a bit in spring.
even indoors i humidify as much as possible.
can you use outdoor/sunlight at all? a stealthy greenhouse might help you cut costs & get high quality light, maintain photoperiod with some screw-in LED bulbs. ?? you may have to grow some corn & peas though
as far as free medium/fertilizer...are you in a rural or urban area? they both have sources to compost from, but compost has a learning curve & takes time & planning if you wanna use it for production(good stuff is 3 years old). i set about to make my own media amendment with a massive worm compost but i'm sitting on 2 yards of it because i need a soil test to move forward. use as much Perlite as you can because it's cheap & light, IMO. to that point, when i bring my 60% perlite indoor mix outside, it needs watering 2x as often as my outdoor 30% perlite blend.
if you can save all of your non-toxic waste you can make your own plant food. i even put my beard shavings in my kitchen waste becuase it's about 50% Nitrogen & slow release. heh. i put every f'n paper napkin & used paper-towel in there too(i mean, it's not like those "waste mgmt" assholes recycle them).
my med intake is about 1/2-1 zip a week and you can imagine the cost of crop failure here. luckily i made a friend who helped me tell corporate dispensary bunk to fuck off for the last & final time. (i ain't in the business of making some asshole a weed-billionaire, or his bought-politicians).
i can see you're already economizing on containers i've been drilling a shitload of holes in mine, diy aero-pot, and am totally satisfied. 5 gal buckets are great.
is there a "craigslist" available to you? check that for free stuff. it's random but you might score if you check it. even used grow stuff cheap.
sorry if the ideas aren't in order to your questions, but i repeat: Perlite. it's about 20$ for 4 cu. ft., you can re-use it until it's sand.
flushing. can you score a Yucca or Aloe Vera plant? or even cheap, plant-based dish soap may help there. i am still looking for a way to use the 1/2 ft of Yucca root in my fridge. it's the amount of raw yucca per gallon i need to figure out. definitely works as soap...even washed the dog with it.
bT for gnats, get a small container of mosquito bits or dunks & soak in dechlorinated h2o w/a tsp molasses. that should last you a while for less than 20$.
i do realize i just told you to spend 50$ & go shovel shit for your compost heheh sorry.
another thought about using natural light if you're in a non-legal area...is there are these small 8"(or so) skylights that use a reflective flexible duct & you have a natural light from your ceiling...& can even control it.
hmm. i'm high. l8r