As a senior SWE, you have already demonstrated technical chops. To progress in your career, what is valued more? Is it business domain expertise within your company or tech stack expertise?
By domain expertise, it can be something niche within your current company you can carve out for yourself.
By tech stack expertise I mean someone who could be really into JVM performance internals.
My personal opinion is tech stack expertise is more lucrative since that is more portable and SWE tenures these days are not very long.
Very interesting question. I think it depends what you are optimizing for.
For promotions and "career progression" I'd say the answer lies between both of those in "Impact." Whatever mix of technical expertise and domain expertise gets you that, you'd want to optimize for. But as a senior SWE you should have at least 1-2 areas people go to you for / consider you the domain expertise, even for interviews if you decide to job switch. But on top of that it gives you so many other benefits too, including opportunities for impact.
For me, I focus on the technical expertise aspect outside of work, and then focus on impact and domain expertise at work. Domain expertise will come over time though from being there longer and ensuring you aren't constantly working only on the same thing.