I sold a subscription of $300/y (accounting software) that I didn't even have built yet (took me a whole month after payment to ship). It didn't matter that I didn't have it, the sales channel worked (i was extremely lucky). Even if it took me 3 months to ship, I would still have had that $300, and I would know exactly how to get another and another when I finally do build it. Most early customers are willing to wait more than you think they can.
The only thing that matters before $1000 per month is marketing, if you don't have a single marketing channel that works then you will never make any SaaS work no matter how much you tweak the code, the website design, the UI, or the pitch deck. That is simple math, 0 leads = 0 sales =$0.
I know most here would think of the previous advice as BS, but the real truth is that most early-stage problems are a quantity problem not the competition or business module or whatever problem.
I did this a few times now, one of which I had a really good product but for the life of me couldn't find eyeballs. Didn't matter in the slightest the product I had. Another time I sold the worst white labeled shit I could find, but the channel worked. It went to 5 figure ARR very fast (months).
How do you find that channel, methodical experimentation. Without changing the product or having a product, experiment a lot with the messaging, the channel, the hook. If you are new to marketing, this can easily take a year of your life before you even begin to understand what the hell is going on. The important thing is to choose 1 channel and keep tweaking, changing the marketing channel is just as destructive as changing the startup idea all together.