Okay, so.
Looking for recommendations from your experience (not just "I searched it up") please.

Looking for a tool to ad-hoc launch a large number of ssh sessions, run specified command/script, capture output and send back.

Unfortunately must accept credentials (user + password) once on start and pass it, securely enough, to all the ssh connections (so no plaintext password in command line or other such madness).
Keys can't be used in this case.

Yes I'm searching, but the one thing that seems to have a similar feel is "pdsh" which's... positively ancient and its ssh support is quite incomplete/broken. And would require ssh keys anyway.

Otherwise basically every post/article suggests to install fashionable central management server of the week and feed it yaml or something.