FYI I replaced the internal HDD in my 2008 iMac with an SSHD (similar to an SSD) and didn't suffer with dust behind the glass etc. Took my time but it made a massive improvement in performance.
Aye, I did similar on my mid 2011 27" iMac but I replaced the Apple 2Tb HDD with a 4Tb drive and didn't get any dust in either.
Oh that's really good to know - I'm actually a little surprised (albeit pleased) that it's made such a difference. When you do the SSD, it'll be even more!
It already is - It won't run Mojave or Catalina so it's a couple of versions out of date but I don't know of any software I regularly use that requires Mojave or CatalinaI imagine the big limitation will be software versions eventually.