Mutifinder gives good advice in reply 2 above.
If you decide to go with the dosdue patch, I'd suggest Mojave instead of Big Sur.
Mojave is the last OS that can still run 32 bit software, so most or all of your older apps will still run on it.
I agree with adding RAM.
I would add ONE 8gb DIMM, replacing the topmost module (while you have the back off to upgrade the hard drive). That will give you 10gb of installed RAM, more than enough.
For a RAM source, I like this place:
DMS specializes in 100% Compatible Apple Memory Upgrades. RAM for MacBook Pro, Mac Pro, Mac Mini, iMac & earlier models, all offered with a lifetime warranty.
www.datamemorysystems.com
THE MOST IMPORTANT step to get more speed is to upgrade the internal drive from a platter-based HDD to an SSD. This is easy to do, anyone can do it.
Go to ifixit.com to see what's involved.
BE SURE TO BUY AND USE THE RIGHT TOOLS (shouting intentional).
As far as buying an SSD, you just need a 2.5" SATA SSD.
DON'T BUY
"FOR SPEED".
Instead, buy "for price" -- Crucial or Sandisk should be fine.
You should also get an external USB3 2.5" enclosure, something like this:
SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)
www.amazon.com
You can use this to "prepare and test" the SSD BEFORE you open the MacBook.
And you can put the old drive in it for use as a backup afterwards.
Suggestion as how to do things (quick version)
- Get the SSD and enclosure
- Put SSD into the enclosure
- Boot up
- Use disk utility to format the SSD to Mac OS extended, journaling enabled, GUID partition format
- Get the dosdude patch
- Use dosdude path to download Mojave (it's in the "tools" menu)
- Work through the dosdue patch to get Mojave installed onto the SSD (not sure what happens here, but if you can get access to disk utility first, use it to re-format the SSD to APFS with journaling enabled)
- Once Mojave is installed, do the setup and use the setup assistant to "migrate" your stuff from the internal drive to the SSD
- TEST BOOT the SSD from the enclosure to make sure it works, and things look as they should
- Then finally... open the MacBook and "swap drives".
- Do a boot, use startup disk pref pane to set the SSD as the new boot drive, and away you go...