I've lost access to my Tumblr account a while back because I enabled 2FA and have since lost the code generator, but back in the day (which appears to be around 2018 or so) they didn't offer the option to create recovery codes, and thus far attempts at paging support have not resulted in anything due to them having a reasonable policy of doubt around this question.
But due to an old automation I had for running a particular highly automatable blog, I had generated an API key. Though it was a bit surprising for me, it still works fully and I could get most data out of my account.
Their API is a nice remnant of the Web 2.0 days. Doesn't seem like it gets a lot of love, but it does work.
I'm not sure I have a lesson to share though I'll say that I'm grateful for today's discovery. Farewell, Tumblr, you used to serve me well. (Note that this link might fall over at any time. When I get around to it, I'll re-export my posts on my own site—I've got them in my possession now.)