Well walking in your shoes I'd say I've got enough data to figure how much time to assign to studying other areas. Look...
I frequently fail in job interviews mainly due to lack of knowledge on the other areas
...Been there done that. Interviews failures (and passes for that matter) are invaluable source of knowledge for what to learn.
After each interview (no matter failed or passed), I make notes marking what areas I was lacking - that makes a solid base for the further study. Then, I just list, prioritize and estimate items extracted from these notes and that makes a guidance on further learning.
- If an item makes a key skill I missed at dream job interview, I try do my best to find time to study it.
- If it's a frequently asked stuff requiring I dunno day or two to grok, I study it too.
- The rest fails into if time allows bucket. If time allows, I study it thoroughly, otherwise try to get a basic understanding or drop it if it doesn't feel important enough
For the sake of completeness: items in my list include not only technical areas but also whatever other skills I may be missing. These can be items like eg, hard time handling post-interview stress or difficulties with paper-and-pen coding - basically anything that I felt was not quite OK at interview.
PS. maintaining and tracking such a list of high-demand technologies might lead you farther than you expect. Thing is, persistence in fiddling with that list kind of breaks the mental block of programming 8 years for a company.
Eg if you decide that you really badly miss something, one day you may decide to change a job to some not-too-appealing position that however gives you an opportunity of full-time experience in desired area. That happened to me twice. Have to admit, each time it felt like jumping into ice water. Nevertheless in the long term perspective it turned out really worth it.