I have a SetApp subscription, and part of that is CleanMyMac-X, and I use that frequently enough for deleting "purgeable" files, leftover tmp files, unused stuff, old crusty bits, etc. Seems to do its job, and occasionally will make a lot of difference.
At the end of the day old machines eventually get outgrown and sometimes need actual physical updates or repairs.
If you're already on an SSD hard drive and are already running 16GB of RAM, you're hitting limits. If you're not, bringing the unit up to those specs will change its entire personality.