I don’t have more than lots of observation, however I don’t think blastr reliability hits more than around 30-50 relays total. Often less.
The 300 numbers gets thrown around a lot, however I suspect the relays being offline/shutdown, rate limiting, now requiring payment, Ip blocking, or whatever else (without stateful retry publishing on failure) doesn’t match the common outcome.
I think generally relay payment support is developing along side - but we have limitations in client UI, subscription payments, being lightning only, etc. I don’t think we even have solid open source relay payment gateways that track pubkeys days until expiry or whatever.