Sidewall stresses (and resulting seepage) are exacerbated by operating with them 'flattened' much more than 'usual'. Turfs with FEL not the ideal for most. (BTDT, YMMV) Right thing, for sure, to find out where the air is coming out. :thumbsup:
Rusty rims can be sealed/sprayed any time a tire comes off a rim. I have good luck with PAM and Fluid Film. (in years, and from one 'app'/wheel)
I agree with the disdain for tubeless tires on our gear, considering that 'yard machine' and 'turf' tires are rarely 'sealed bead' like auto/truck tires are.
'Tubing' mowers and such has been my std since using riders, but low air pressure causes tube squirm/scrubbing/leaks, and 12" & smaller rims are a bear
w/o tubes, so still a good idea to check pressures often. Tire says "Max 15 PSI and your butt/back say 'nothing over ten'?

You don't have much air to lose. ("Sports"/basketball pressure gauge for under 10 PSI.)
Were I the OP, or in similar straits, I'd start with tubes. Clean, easy, not messy now or later.
From experience, I'd leave Slime to tubes only, and only foam-fill a tire whose eventual replacement would include a 'new' rim too.
(I wouldn't blame Kubo or Titan for something so well 'engineered' to be a problem. :confused3
