I too am looking at the staff route, and have mostly a front end background.

I hope this does not seem judgemental, but I think you still have a ways to go. You are talking about externally directed learning and being taught how to "do" architecture - these are things I would associate with the mid to senior transition.

My job - which is still technically senior engineer - involves me doing work like an in house code graphing tool that takes the a project's AST and allows it to be interrogated by ad hoc queries.

Similarly, I am writing a system wide application graph using PegJS to prototype a custom query language. You can use these queries to visualise the service map and how the entire system architecture works, including databases and messaging.

I'm not sharing these things to flex on you, but to try and indicate what the work looks like as you get towards this level. It tends to be research led and about lots of meta-work. It is usually full-stack. It is often somewhat speculative and always self driven: there is no book on how to do it; at staff+ imo you need to be the person who would be writing the book if you were not otherwise busy.

At 6 yoe I would focus on being a great senior engineer, learning in a self directed way, and broadening your skillset. To do staff+ you have to be a generalist, so start thinking about what excites you outside frontend dev