OP:
If you'd like to keep what you have going a while longer, as others have mentioned, just add RAM (2 8gb DIMMs would probably be a big help), and get an external SSD from which to boot and run.
Since you already use SuperDuper, setting up the external SSD is ... well... it's "child's play" on a Mac.
For RAM, I've had good results here:
Shop
www.datamemorysystems.com
(looks like 2x8gb is on sale)
For an external SSD, I'd recommend the new Samsung t9:
(shortened amazon link)
1tb size should be "all you need" -- you never told us what size your current drive is, it's a fusion drive, right?
So an outlay of about $160 or so should get you fixed up and good for at least another year or two (or perhaps even longer).