FYI, this is fake news OnlyKey has been around since 2016 and has been open source the whole time. Solo was launched in 2018 and claimed to be the first FIDO2 open source security key, this was only true because at the time OnlyKey wasn't FIDO2. OnlyKey was the first open source security key. Also Solo isn't even a viable alternative to YubiKey as it doesn't support challenge-response, static passwords, or OpenPGP. OnlyKey does support all of those things.

Solo is great, I would personally recommend this key if what you need is a 2nd factor. If you are looking for more features like password management and additional 2FA options then OnlyKey is a good choice.