may I use my external hard drive to store other data after using it for Time Machine?

That is not recommended.

For one thing, where will you store the backup copies of those extra files? on the same drive? Then one Drive failure would then take out every copy of those files.

Also, Time machine works better and can hold a longer History when more space is available for older (than most recent) copies, and Time machine can reconstruct your drive as of any date still in the backup set.

I'm doing this myself by using Disk Utility to create a new volume on the original Time Machine (Time Machine originally dedicated the entire SSD to be a Time Machine). It mounts like a separate drive on my desktop. I'm storing old files (files that I don't plan on saving to my Mac and so they won't be saved by Time Machine) in the new volume while using the original volume that is a dedicated Time Machine storage device. Yes, I'm doubling up dependence on that SSD. But this is all somewhat a temporary stage as I want to eventually get all my files off of old spinning hard drives.

Time Machine works at low priority in the background, so as not to interfere with your important foreground work.

Time machine gets no extra benefit from a drive that is fast.

Punchline:

spinning drives that are not especially fast can be great Time Machine drives.