For early stage companies, how much do you prioritize making a formal brand guideline?

It's not that important at all. If you don't have a product with Product Market Fit, then you should be building and talking to users. Once you have PMF and a sales flywheel that works, then you can work on brand guidelines.

I'll try my best to stick to a color scheme but outside of that I'm not really prioritizing it!

Just bare basics and refine as you grow At the beginning of my startup, once I found a name (so difficult lol), I designed a super simple logo, picked a font, and 2 colours. That was enough to create simple marketing content and a website.

Early-stage companies should prioritize creating a formal brand guideline if they want to create a consistent and memorable brand identity.