I have not read through all the responses, but I’ll throw in my experience. I’m in a similar position of paying off student debt, not as much though, but still wanting to progress myself and my horse.
I found a trainer that I was curious about working with. The first 8 lessons, I paid for. I established myself with him, showed him I was able and willing, and as time passed I informally asked him if he needed a groom for an upcoming show. At first he was a little shocked because most of his clients are Amateur riders and therefore can’t help in that way - I don’t think he realized I have held a pro card since I was 18
After that offer, he started having me come out to work at the farm when he was out of town. I work horses, clean them up, etc. I have groomed at shows for him, braided his hunt horses, done whatever the heck he needs me to do. The last time I went out there, their stall cleaner was injured and it was a group effort to get all the stalls done. Originally he said ‘I’ll pay you X amount per hour’ but it has now become ‘when you want to bring your horse out for a lesson, just let me know’. I have never received cash/payment from him, but I have also not paid for anything but my own show fees since last summer.
While it ruined my 3rd attempt of regaining my amateur card, it’s what I need to do right now to progress.
The main difference between my trade off and yours, as has been mentioned above, is the time commitment. When I walk away from the barn, I’m done. I’m not going back home to edit pictures for hours, or (insert other fancy design things here). It does cut into the time I get to work with my own horse, BUT every time I work with one of his, I learn something for mine own, so it’s never time truly lost. And, I tell him that. It’s absolutely a privilege to learn from him. But for you, it may be a major time chunk to be able to do what you are offering, and is it worth it? Or, like was said above, is it more worth it to get the money from an actual client and leave the horse stuff alone (and retain you A card )
ETA: I only take 1-ish lesson a month currently…so, the work to lesson ratio is very much in the trainer’s favor. I take a lesson and go home and do A LOT of homework. When I come back I want to be able to move forward and I know his time is very valuable so I try to respect it.