This isn’t exactly a comprehensive answer to your question about what’s morally permissible and what isn’t, but my view is that if it’s going to be a huge hassle and expense for you to avoid flying, you shouldn’t make yourself feel awful about the fact that you’ve done something you regard as less than ideal. I would just donate to Clean Air Task Force (probably an amount that will more than cover the impact of the flights in expectation) and continue trying to avoid flights in future when feasible if you think that’s something you want to do.
This isn’t exactly a comprehensive answer to your question about what’s morally permissible and what isn’t, but my view is that if it’s going to be a huge hassle and expense for you to avoid flying, you shouldn’t make yourself feel awful about the fact that you’ve done something you regard as less than ideal. I would just donate to Clean Air Task Force (probably an amount that will more than cover the impact of the flights in expectation) and continue trying to avoid flights in future when feasible if you think that’s something you want to do.