Some good news here. Just received a batch of emails from Ryanair / Laudamotion / Malta Air. Vouchers previously offered will now revert to full refund if unused within twelve months. Balance on partly-used vouchers will be either refunded or replaced with new vouchers. Those options address my previous concerns since I am a frequent user of Ryanair services. I was worried that since I had a substantial number of refunds due I might struggle to use them within the window available between the resumption of normal travel and the expiry date of the voucher(s).
My only slight criticism is that each seperate email does not specify the booking reference to which it applies. However, it does appear possible to discern this from the long sequence of digits in the acceptance link. I've got a whole batch of these rolling in!
I'm now content to allow Ryanair this medium-term loan on the understanding that my vouchers won't expire worthless before I can use them. Sensible outcome.