It's very nice that SQL dbs have the same bones such that a migration between them is even reasonable for a small team.

That said I've led such a migration, and there's no such thing as "standard SQL" in real practice. You're always going to need extras or optimizations.

Rel itself looks really nice, but I have a hard time imagining trying to justify a new query language in business terms. Breaking with SQL completely doesn't seem worth it compared to simply extending or restricting the language like every engine does anyway.

Rel seems to offer more than SQL, it's just hard to assess the value.

A SQL engine whose dialect enforces unique rows and doesn't include NULL isn't even so different from standard.