To paraphrase: it seems that you are noting that you don't have time outside of work to pick up something new and don't want to start (financially) at "entry-level", but you do want to work in a new field nonetheless.
The situation that has worked for me in the past has been working for a company where the engineers "wear multiple hats": e.g. a contract engineering company. In a company with a fixed staff that does multiple tasks such as building hardware, developing firmware, creating web applications/site, developing software on multiple platforms, etc., people tend to be more generalists. The types of products that a contract engineering company will be hired to create can vary greatly...
Working in such a space opens one up to new opportunities and broad knowledge (and sometimes you can be forced into unfamiliar territory, which makes one "swim or sink" - a definite incentive to learn new skills quickly). You may be doing what you are most competent with most of the time (especially early on), but with drive and interest it is possible to become comfortable in seemingly different areas.
Then again, if you are in a larger (non-contract engineering) company there may be opportunities to work on other tasks outside of your general field of knowledge. Look for something relatively small and manageable that needs to be done, request the opportunity, and then do it quickly with as much competency as you can muster. If you prove yourself once, there may be more opportunities you can claim. You likely won't get what you don't ask for, so be persistent.