I have an old Mac Mini (2009, I think) running as a headless server. I did fit an SSD. Won't run the lastest MacOS but to be honest, it's shockingly good considering its age. I also have a 2011 Macbook Air that still more than capable for surfing* and writing. SSD is the key, the OS is optimised for it, and the moment you have one, spinning rust seems glacially slow.
Yeah, avoid Office and Adobe, they max out most configurations (I'm never clear why, they do little that previous versions don't). My mothership Macbook is getting old now (c2017), but despite heavy use with monster spreadsheets and design packages, rarely uses the full 8GB RAM it has on-board. I doubt I've ever got through the 24GB on my iMac (it was on offer).
I've never bothered with any kind of clean-up, MacOS should do its own maintenance.
*I was reading the other day that someone finally managed to permanently beachball his Mac Pro by simply opening 6000 browser tabs.