Wow, this sounds eerily similar to my situation, though I was only there about 3 years.
I left to join an AV startup and it's amazing how much more I've learned and accomplished in a few months versus the time at Google. Things at Google move slowly, and the amount of work per person is relatively limited. Also, all the complicated infrastructure or codebase decisions were already made for you, or is being handled by someone L+2 at least and outside of your purview.
Edmond Lau's The Effective Engineer talks about this, except to the extreme that he wanted to do and learn everything at Google, and even then he left after ~2 years after he felt his growth was slowing.
I think for many, being at Google for a few years will give you invaluable experience, but then severely diminishing returns on growth and practical experience unless you're one of the lucky ones who gets promo'd every year or two.
For your actual question, though:
>Now that I'm thinking of jumping ship to other interesting companies, I'm having serious doubts that I really learned what I should have learned during all those years.
Having Google on your resume always helps get people interested. Having experience working on big teams with big codebases is also something not everyone in this industry has, and there's value to it, even if you'll initially scratch your head at how to build without Blaze.
>Especially since I'm considering companies with a higher hiring bar than Google.
Curious why you're confident in that statement. I've found that Google has a much higher hiring bar to the actual required skill -- they basically seem to hire as though everyone will work on GWS, when in reality many are just copy-pasting CSS and BUILD rules from another project.
On the flip side, many more interesting companies have lower hiring bars relative to the job requirement. It's harder to hire good talent when you're not FAANG with a pipeline right out of the Ivy League.
>How can I keep myself accountable while I'm still at the company to deeply learn the FE/BE technologies to be better prepared for other companies? Should I start by preparing a checklist of technologies and dive into each of them for a month and continue from there?
Well, I would focus LeetCode, tbqh. That's still the standard. But whatever you want to do, focus in on technologies used in that industry/job role. Do some side projects. Maybe take some online classes. I think you'll find the practical experience requirement to be lower than you think. People can generally learn the right technologies, and companies know this. It won't be a big deal unless you're a frontend-only SWE who suddenly wants a ML role or something.
Lastly -- feel free to DM me, I use this handle on twitter and gmail, happy to help, especially if you're curious about where I ended up.