Well..I've been looking at different CAs but I don't understand about this. Would SSLForFree, ZeroSSL and CACert CAs have the same problem as let's encrypt?
In /dev_flash/data/cert, your have a series of certificate files, it's the trusted store basically.
The trusted CAs are all there, all the newer additions in recent years are found in CA_LIST.cer.
If you look there, you will find both let's encrypt CAs, the DST X3 and the ISRG X1.
No idea why the latter doesn't work..
For other SSL certificate solutions, in theory you would need to check that they use RSA SHA 256 and whether they are cross signed with a root CA included in the PS3 trusted store but ideally in practice what you really need is a test page for each proposed certificate, so you can be sure it works on PS3, something that few SSL issuers/resellers seem to offer...