As a product designer, I personally favor Sketch and have used it daily for the past several years. As long as you're going with Sketch or Figma you're pretty much set. I could write essays about the positive features of both.
The reason I prefer Sketch is that I don't like people looking over my shoulder while I'm in design mode and seeing multiple people inside the document I'm working on just makes me feel anxious and less inspired to experiment. Plus, I have found more resources for Sketch than other design tools.
At the end of the day, pick the tool that works right for you in your context.
@gruen covered a lot of great stuff regarding workflow, but I'd like to add my insight on tools as someone who's worked within a couple of early-stage startups.
The most important thing is to make sure that you have tools that people in your startup will actually use. Too many tools? Too complex? Nobody will use them.
I've tried pretty much every tool out there within the last few years and have found the best combination to be GitHub, Notion, and Slack. This has been echoed by companies like Gumroad and Sketch.
GitHub ends up managing milestones for the software, as well as tracking issues and the overall development work of the product. Do note you will need to pay for GitHub to keep your repositories private unless you're okay with them being public.
Notion can really act as your company's HQ and track all of your non-development tasks and goals in one place. It's very easy to turn kanbans into lists and many other views. It may be a little overwhelming for the first hour, but after spending a little time with it, you'll see the potential. This can be free up to a certain point.
Slack's free plan is more than enough unless you plan on storing files and need chat history. This is where I highly recommend implementing a standup bot so that every day or week everyone is updated with what everyone is working on.