How important is SEO in the early stage?

I think that SEO is a long-term strategy. It depends on your market but it's hard to see the results the next day.

There is a great free workshop from Corey Haines - How To Get Your First $1k, $10k, and $100k MRR.
https://learn.coreyhaines.co/how-to-get-your-first-1k-10k-and-100k-more

It was very helpful for me, not only in sales & marketing.

SEO takes a minimum of 6-12 months to show results. However, I'd argue that you should be doing two things from Day 1:

1. Determine if SEO is a viable strategy for your business.
2. If it is viable, you should include SEO requirements as you go.

SEO is not a one-time activity - it's something you do continuously as part of your marketing and dev work. If it's not a viable strategy for your business, you'll simplify your life by not needing to worry about it. If it is your strategy, you'll be more successful by including that thinking early.

Don’t think SEO as a separate task from your other activities. Proper SEO help you understand your customers and competitors growth strategy.

Start with on page seo and move to off page. If you don’t know what’s the difference, try checking Moz.com for their guides.

Then you monitor and understand your competitor strategy with some tools like Ahref and Semrush , you can also copy their method and it’s not wrong.

SEO save your time and budget for longterm.

Will it bring you sales on day 1? No.
But you won't have enough sales after 6 months if you don't start straight away.

Google rankings are all about content now, and that ol' SEO juice takes 3+ months to build up and be useful.
Start soon.
Write useful content, share it in places, build some backlinks - after a few months you'll notice a strong %age of traffic coming organically, referred to those pages.

I get multiple organic signups a week for Trolley, my payment service, through organic traffic to useful blog posts I wrote months ago.