Residence card of a family member of EEA national

You should apply for the EU settlement scheme. It is free of charge. From what you've told us, you should get permanent "settled status," but there is a chance that you will get "pre-settled status," which must be upgraded to settled status after a period of time.

Note that your problem isn't that your "visa" is about to "expire"; you don't have a visa. You have a right to reside in the UK under the Immigration (EEA) Regulations 2016, a right which does not expire. (It can lapse because of a change in circumstances, but not because of the passage of time.) The residence card you have is merely evidence of that right; it's not actually required for you to be in possession of the card, let alone for it to be valid.

Your problem, rather, is that the 2016 regulations are expected to be repealed, in connection with the UK's withdrawal from the EU, at the end of the transition period. That is presently expected to occur at the end of this year. The settlement scheme is the legal mechanism that allows you to remain in the UK beyond that point.