I also have and still use a XS Max since the launch 3 years ago. I'm also thinking of upgrading just because its always fun to have a new phone especially after 3 years, but objectively when I look on my XS Max, it still works extremely well. Still very fast and snappy and I don't think I'll see a huge difference in performance between this and the 13 pro max in almost all situations. My battery heath is at 95% so another reason to just stick with this phone.
The only gripes I have with this phone is even with the battery have good health, the phone is more warmer than before and battery drain more easily by doing basic tasks. Its still last for the day, but I feel iOS is less optimized for this phone since iOS 14. Also, having more ram and a better display is always a plus. 4GB RAM is not enough in 2021 and apps don't stay long in the background like it was 2-3 years ago.
So I don't know what to do. I have the feeling next year iPhone will be a bigger upgrade and will worth it more than this year, so I'll probably listen my head and just stick with my XS Max until then. Like I said, its still a very capable phone and I'm using my iPad pro most of the time so its more about me wanting some change and have a new gadget to play than anything else. Its not like previous upgrade where I really needed a new phone because in the iPhone 6s and below era, phones performance and battery degraded quickly and after 1-2 years I was more frustrated with my phones than anything.