How do you recommend anyone who wants to start a startup in open source as solo developer and want to monetize?
I answered this elsewhere! Check that out.
How do you keep up with Work and Life balance?
Create a schedule and be disciplined to stick to it.
For example, I only accept recurring meetings on Mondays and Fridays (with a couple exceptions for good reason). If another recurring meeting needs to be scheduled, another one has to go. I use T - Th for one-off meetings and to focus on other work.
Also, I have a mandatory policy for myself (and I highly encourage it for others) to take a 1 hour lunch break. I have a calendar block so no meetings can be scheduled during that break. Of course, I'm practical and make exceptions once in awhile, but I'll just shift my 1 hour. I almost always take a full 1 hour break every day.
I built a home gym for myself. It wasn't cheap, but can you really place a price on personal health (for money that would be spent on non-necessities anyways)? This means I can interrupt my day to work out for 30 or 45 minutes, and I don't have to commute to a gym. I can shower at home right when I'm done. It is very efficient and I work out much more because of it.