An excellent way to spot these opportunities is to see what services big companies give away for free. These companies feel threatened by these things and need MORE competition, not less (look at what happened to email hosting for a good example of what happens if people don't compete).
When I see a company offer me something for free, I immediately see what it costs (I want to understand the value I'm getting, at a bare minimum). What I see out there is a thriving economy that I never would have thought to look for if it weren't advertised as "free."
I kinda _depend_ now on syncthing, organicmaps (openstreetmaps), and maybe even Firefox's offline translation service (just to name solutions to those concerns you also mentioned).
Maybe the common theme here is "offline" (so that you are off the loop of big tech's services), and being offline has other positive consequence anyway.
I too built things my whole life and have years of 'failure' on my back but I live of income I make on 'niche' sites no major company is ever going to compete for. No unicorns, no million dollar business but honest money for honest work.