Managing up - strategies for a good relationship with a manager who was the founder of the business?

Not a manager myself, and never have been, but I am at a high level individual contributer role.

I am newish to the role, almost 1 year in. The company has hit a huge growth spurt the past couple years - going from 20 employees to 50+. My boss is the co-founder of the business team years ago, now the CTO and developed a lot of the original tech stack.

We are hiring a lot of smart people (me included, I'd like to say) and my boss comments, not always positively, how talented everyone seems

He doesn't appear to like the leadership role he's in, and often takes on little side coding projects. He struggles with communication and taking feedback, and can be very prickly when we want to make changes to the legacy stuff he developed. Otherwise, I like working for the company quite a lot, and like him as a person.

He has all the appearances of being burnt out. And I'm failing to get him to engage with the much needed upgrades I see our systems needing. Last month I pushed harder than normal for changes and I feel this damaged our relationship, and that he resents me slightly now.

He's also recently started mentioning in our meetings how he could have personally done the projects we work on. I'm not sure how to approach this employee-employer relationship because being direct is often a mistake, we find.

Any strategies you managers find helpful for navigating this sort of leadership personality?