I was then wondering if it would be fair and respectful to bargain a higher salary (20%+) given that I have existing knowledge that will add direct value to the start-up.
There is nothing wrong with trying to negotiate a higher salary, but you might be overestimating the value of your experience.
Let's look at your experience and assume with everything else ...
I encourage you to look for openings at small businesses. In a large organization, most employees are disconnected from understanding "where the money comes from." In a small organization, having employees that an understanding can be a very valuable asset for a small business.
About 12 years ago, I applied for a position at a 20-person company. ...
You didn't quite phrase the manager's fear right:
Now if I were a hiring manager, I would (perhaps) be impressed but
also very worried: if this guy maintains such big and important
software, how is he going to have the time to come work here?
That's not the fear. Let me alter it a bit for you:
Now, if I were a hiring manager, I would be ...
Try job fairs.
The first rule of getting a job is to tell a story, and you have a great story to tell, but no one is hearing it because they never get past your resume. So go to the places where you can lead with the story. Find the job fairs in your area that are looking for people like you, go to them, and hit up the small companies. That will let you ...
How do I credibly convey that I a) am sufficiently accomplished and
Some client testimonials? List a few projects you've done (even if it's only features in your SaaS) that demonstrate your capabilities. Some would tell you to show code but I've never had to do that to get a job (except when I first started in the industry).
b) actually available for full-...
This is a good example of the "exactly" argument. It can be found in any book on closing sales under "overcoming objections". You turn the objection into the reason why your argument is correct.
"Your product is more expensive"
"That's exactly why you should buy it, we are higher quality, and thus more expensive, but a ...