I think that's it. All are very saturated (I know .. 10 years ago people also said that).
Tech-enabled business seems to want websites, cms mostly which has plenty of off-shelf. Niche markets, most of the time, are too niche. Not worth building apps for.
"Yet another x is still good if that market is huge" .. doesn't sound good to me, because at some point it's just not good.
Thanks.
Those are good ideas to start with B2B apps.
Not sure it's a good idea but:
an app marketplace where companies post some needs (problems) and apps in the market only have a trial plan. So companies discover more solutions, founders get pmf?
These are 2 extensive maps for the app territory. They may at least offer you some keywords (to google)