Fat experiences

At this point you need to get yourself a scooter

Track your movements, see how far you can go before tiring and further before feeling like you will keel over. Look for places where you can lean or sit if no public seating, like a ledge or even school steps if you feel your back about to give out.

If taking Rx or painkillers occasionally try to space them out and lessen damage long term. Wall/back strengthening exercises can help, more so on an empty stomach.

Like some have mentioned before, a scooter/electric wheelchair would be a good investment especially if you're looking a reaching the 500 mark. I'd also look into getting an apartment or house on a single floor, has a walk-in shower with a strong enough bench to sit so you don't have to stand for too long, an electric adjustable bed to help you get up or stay seated up.

It's sounds very expensive, but that's the cost of being a feedee. I'd recommend starting an onlyfans and /or start camming for the funds needed.

However, I'd recommend (and you might not like this...) to walk more and lift some weights / strength bands. It will go a long way to help with your gaining goals if you stay active and add some muscle to carry your growing frame