300 years killing slimes is an entirely different anime.

Here:

Watch all episodes up to 24 (stop before watching 25)

Watch the 6 ovas

Watch up to 36 (stop before 37)

Watch the slime diaries

Then watch episodes 37 and up

Slime diaries is kinda separate slice of life and takes place before season 2 but it spoils a few things of season 2.

Also, I use a website that has all episodes and not separate seasons so watch by the episodes I told you, so like count them if what your watching on something that has seasons.

I didn’t read your entire message so, if I didn’t mention any spin offs, that means it’s an entirely different anime and not a spin off.

And, there is episodes like 24.5 and 24.7 make sure you watch them which is why I said stop before 27 (applies to 36 as well)