Unfortunately SoloKey doesn't work as an OpenPGP smart card, which means it's not a real substitute for a Yubikey. I haven't had any luck with resident FIDO2, either.
The Solo team believes that other functionality such as PIV overlaps with GnuPG use cases, so that OpenPGP isn't a priority, and their work on that functionality appears to have stopped in 2021. That's too bad, because OpenPGP's network effects far outweigh its pure functionality, which means a technical substitute isn't a substitute.
OpenPGP is arriving, will be available via firmware upgrade.
Support for openpgp is almost there: https://github.com/solokeys/openpgp
Note that the ed25519 is not as easy as it seems, ssh agent doesn’t support it yet. I wrote a couple notes/details in the somu campaign you linked, towards the end.
SoloKey doesn't support everything this does. It is primarily a U2F key, and OpenPGP support is still WIP[0]
FYI, openpgp and ssh are already in progress. The stretch goal is mostly to carve out money for ft development.