i like the idea of using a low-code tool like Anvil:
https://anvil.works/
that said, i've never used that particular tool or really any of them, but Anvil changed their pricing recently to make things more startup-friendly, imo.
You'd still face all the same questions tho.
There are anvil-type partner agencies that presumably build saas apps pretty regularly. maybe search out some of the 'no/low-code businesses that made it'-type sites that seem to pop up every once in a while. a lot of it is hype, imo, but... possibly still legit.
I don't know if HN has a "What do you think about this type of app?" post type, but maybe you could put it in 'Ask HN' without getting downvoted to hell?
That can bring good and bad, too, so not sure it's all that useful. Some people love to hate.
But you might be able to find someone who is particularly interested in your idea and wants to help.
I have a couple of folks I use for relatively inexpensive development of saas apps using Laravel (a PHP-based framework), sometimes with various plugins - they're small operations so that's good, but it's not multiple people with a project manager, etc., so there's just going to be more work you have to do in that situation - which can be tough if you're trying to take care of your day job, too. Feel free to ping me.
Anvil can seemingly do a lot, but might not be able to do everything -- like, it might not be able to easily do team-based or multi-tenant saas (like Trello), whereas it prob can do a regular/simple saas like your personal gmail account easily enough. and building an app - like a native iphone/android app - is a diff ball of wax. $50k alone type of thing.
i use balsamiq for mockups and it's great. i use the 2-project plan, and retire/unretire projects as needed - the higher-level plans are too expensive for my blood. there is nothing else that comes close to balsamiq, imo. everything else is too complicated, or too garbage, or too pricey, or too whatever.
i usually start with pen (or pencil) and paper, a notebook, and start making notes, drawing screens, etc. then i take it to my dev and say, "here, please do this, and please give me an estimate of cost and timeline."
my latest project built this way is this:
https://navbarlinks.com/
i'm pleased with that outcome, and i may do further dev on it, not sure yet.