after putting in stratum+tcp://stratum.slushpool.com:3333 for the URL, my user, and a random password
I am rusty too, but at Slush you may need username.workername (I don't know how it handles missing worker).
https://slushpool.com/help/bitcoin-setup/?c=btcSince tell "Auth Failed" I'd go by this first (also log in and
create a worker).
But you may also have surprises on low hashrate, but that may translate in more disconnection than useful hashes.