I did stovetop 3 gallon batches for a while before I got a Brewzilla. The general rule of thumb for BIAB is, your kettle should be twice as big as the amount of wort into the fermenter. So a 20 quart (5 gallon) kettles is perfect for up to 2.5 gallons of wort. Maybe 3 if your grain bill is...
I get frustrated too because I brew 2-3 gallon sizes. My way around it is to download the Recipe Sheet from the kits (there's usually a link on the kit's webpage) and get just that quantity of ingredients at the homebrew store. If I'm feeling extra, I'll find other kits with similar...
I have 2 uKegs and this is normal for me. I use one 16g cartridge and leave it at max PSI for 2 full days, in the fridge, swirling whenever I can. Then I swap in a new cartridge on day 3, purge, and set it for 5 PSI just to dispense some beer. If it's good, I just leave the second cartridge...
If you're okay with aluminum, find a local Mexican grocery store ("supermercado") and ask for a tamale pot/steamer ("vapoera"). I bought a 10-gallon pot for like $40 and it comes with a lid and a steamer insert that I use a false bottom of sorts.