You might check out plausible we've had good luck with them. Mixpanel gives you a ton of info but will take a lot of time to set up and in the early stage you'll be spending so much time on other things, you won't be able to use all the data Mixpanel is giving you.
Google Analytics predominantly but if you might want to think about the following for your specific question:
- Email Analytics - how you are (or not!) converting customers
- Link Sharing Analytics - I use a custom url shortner to help me on this one, means that I can brand campaigns and track how well they perform.
Google analytics for quantitative data.
Smartlook Qualitative Analytics for qualitative data.
I recommend deploying both using google tag manager (GTM) so you can set up all your events from GTM and easily propagate them to different apps.
Mixpanel and Amplitude are nice for product analytics:
- Event analytics
- Funnel breakdowns to improve onboarding
However, they are a bit overkill for early-stage startups. Also, pay attention that they have different pricing models. Mixpanel is charging for the tracked events while Amplitude is charging for tracked users. So depending on your app usage, one will be better than the other.
For validation of ideas/hypotheses, I am not sure if Google Analytics or any other tracking can come in handy here. IMO if you want to validate ideas, you instead want to learn how to use Google Search Trends and do Reddit or Twitter analysis to understand a market fit for your vision.