I also have done work for the furry community, and I make it clear from the very start that I only allow a certain amount of revisions for each step of the process (5 for sketch, 3 for lines, 3 for flats, 2 for shading) before I charge extra as a part of the contract people accept when they send in the form to commission me. If you request major changes to the composition that makes me have to change things at the sketch level, and I'm at the colour phase, you compensate me for the time I already spent and we start again. I never tell anyone they CAN'T ask me for changes at all, just that you're entitled to a certain amount of fixes at every step before I charge for additional ones for that step.

If my clients later don't like my terms or refuse to pay for the extra changes, then I communicate that I have to continue on with the process, or they're welcome to a partial refund minus the cost for the work I put in already, and they can keep what I already did for them to finish for themself or find someone else who can finish it for them.

I don't blame people for being exhausted because that's what happened to me before I added in those clauses to my contract, but if you get paid for a job, it's your responsibility to do it right. If you don't do it right, refuse to fix it to make it right, and then don't allow people to edit it as necessary to make it right when it should have been done right, that's really terrible business sense.