Spend time reviewing PRs and making thoughtful suggestions on how to improve things. Don't bikeshed on technology -- like mongo vs postgres, angular vs react -- if the code is written in a clean way, those technology decisions don't really matter that much.
Read "The Prince".
I have to say though, what sort of technical lead is letting the juniors define anything? Their opinions are worth listening to, but it is the role of the leader to make the decisions.
Honestly he sounds like a shit tech lead - a leader should, well, lead.
In the grand scheme of things, the choice between Postgres vs Mongo for a silly website is insignificant. Don't lose to much sleep over it.
"Herding Cats: A Primer for Programmers Who Lead Programmers" is a good book for psychological issues like this.
Building a team of noobs takes a lot of work. That kind of work, I'll only put in a startup where I've ownership.