As far as staying relevant, I’ve developed a network of trusted local external recruiters and I speak with them to see what companies want.
When I’ve been looking for a job, I’ve found jobs where I have the listed “must have” skills and I want the “nice to have” skills. Rinse and repeat.
I don’t do non work related side projects. I will work extra hours and volunteer for a low risk project using new to me technology just to learn.
I want to say that I would change jobs if I saw the technology that my current company is using is out of sync with the job market all other things being equal. But, every job change I’ve done in the past decade has come with both more pay and more resume building opportunities.
Now after job hopping and following my own advice for a decade, I’ve reached close to the local market (Atlanta) maximum unless I start working for a consulting company (not contracting/staff augmentation).