I've been using this tool called tumblr-utils to back up my tumblr blogs. it creates a locally navigatable archive of a given tumblr url's posts, which is more convenient than the post soup you get from tumblr's native blog export feature.
what that means is that I have a folder on my computer with the name of my url with an index.html file in it, and when i click on that file to open it in a browser I get a simple page with a list of years and months. selecting a specific month will send me to a list of the posts i made or reblogged in that month, similar to tumblr's own archive page. the contents of the post including images are stored locally on your machine.
It can also make a separate index file that organises posts by tag, which is great if you're a consistent tagger, but it will list every single tag you've ever used so it can take a while to find the tag you're looking for in the list if you're a habitual tag commentator. generating the tag archive also takes a while depending on how many posts have to be processed.
you can make it back up any blog as long as it's not set to private. I have backups of both my main and sideblogs and it keeps them in separate folders.
it's had some trouble going all the way back to the start of my main blog in 2012 just by sheer volume of posts, but by making it fetch posts from one month at a time I've been able to go back to 2015 (that's tens of thousands of posts), which was good enough for my purposes.
it might be a little scary to use if you've never touched the command line before, but there's both text and video instructions to set it up and using it is just a matter of typing the command and letting it do its thing in the background.
This document has a really good guide for setting it up, along with some other options for backup. I've been using tumblr utils for a while myself, and I run an incremental backup once a week.
