i would not recommend deleting old content.
perhaps if you have a funny picture thread from 10years ago, sure, kill it. but on-topic things, don't do it. your future self will thank you.
convert to webp with xf2.3 optimizer. (built in)
throw the attachments on a bucket for cheap. (just need to set up the bucket and add a few entries to config file, and then bulk move them with a simple command line)