Old iMac what should I do?

+1 on getting rid of Catalina, you bought a 100% hack which will fail in many ways.

Also, who says you need to buy a new iMac, there are many alternatives available to you. For example:

• Buy an Apple refurbished Mac directly from Apple. They are available at Apple.com, then navigate to the bottom of the page and search for the Refurbished section. These machines are identical to a new Mac, have the same warranty as a new Mac, are eligible for AppleCare and can save hundreds of $$$.
• Don't buy an iMac, buy a Mac Mini, buy a M1 based iMac or MBP or other lower cost solutions than a new 27" iMac. Any would be a HUGE upgrade over what you have now!
• Finance your Mac from Apple. If you buy any new or refurbished Mac from Apple and you have decent credit you can get 12 months same as cash financing. If you can afford $250 per month you can buy a very nice Mac Mini and then use your old Mac keyboard and mouse with the new Mini. Add $100 for a display and you are in business!
• Buy a used Mac from a reputable reseller such as Macsales.com. NEVER EVER buy from Ebay, Craigs List, Market Place, Amazon.

+1 for reverting to an older version of MacOS. Even ignoring hack reliability there are still problems with Catalina:

• 4GB RAM is the bare minimum to run Catalina. Even if yours has an SSD upgrade being stuck with 4-6GB RAM max and a Core 2 Duo processor is going to be a slow. It is much better to run an older OS well than to limp through a newer OS.
• Catalina removed support for all 32-bit applications. If these are older apps you have been using that is probably why they no longer work so you would either need to downgrade anyway to keep using them.

If 10.11 is too old for your apps and you truly can't afford anything newer you could try switching to Linux. I've heard good things about the Mint distro. However if going Linux do create an El Capitan USB installer first to make reinstalling MacOS if/when desired easy.

Get rid of macOS Catalina ASAP!

It was NEVER designed to run on your old Mac. Revert immediately to OS X El Capitan per How to create a bootable installer for macOS - Apple Support, then use the installer to erase your startup disk and install OS X El Capitan.

Backup your data first, as performing the steps will erase your startup disk.

Jack

I wholeheartedly agree with those who've posted before me and suggest you begin saving up for a new Mac. The model you have is older in human years than I am and that's not good. Make sure you maintain a current and complete backup at all time in case the iMac has a cardiac arrest.

Apple is holding an event next Monday and I'm hoping they will be announcing the new 27" iMacs (rumored to now be 32"). We'll wait and see. Apple makes it easy to finance as you're already been told. Do consider it.