Does anybody know of adapters that allow you to share attachments between mixers like Kitchen Aid/Cuisinart/Electrolux etc?

Great question, Chris! Unfortunately, many manufacturers design their own attachments for this very reason so you can't easily swap them around different brands. I agree with Nancy's suggestion to try donating the attachments, I'm sure there's a local Buy Nothing Group that would welcome them. And if you're not sure what that is, here's an intro: https://food52.com/blog/26376-what-is-a-buy-nothing-group

Never heard of interchangeable mixer parts - unless a company changed name and parts were usable in both brand name appliances.
Guessing you've got lots of KA attachments they cost a fair amount but you won't be able to use if you buy another brand mixer.
Try selling them second hand or maybe give them to organizations that provide for people in need setting up new homes...