> In my case, that's come with a never-ending need to constantly retool: new languages, new SDKs, new device types, new APIs, etc., etc.

I think over time those things are not as significant as the changes in disciplines (titles?) or verticals. E.g. Changing from an embedded SE to front end UX is more difficult than the languages or tools involved.

Edit: is it harder to retrain from front end to embedded? I've traveled the ≈embedded to ≈ux road (via ≈applications). From ≈employee land to ≈owner land.