What's free forever and won't hold my notes hostage the day I stop paying? App Question

Separate the two questions, because they're independent and only one of them matters. Is the app free? Are the files yours? An app that costs money and writes plain markdown into a folder you control is far safer than a free app keeping everything in its own database. Judge on file format, not on price.

Open source with a local file store plus your own backup is the belt-and-braces version, and it does cost you convenience. I sync mine with a self-hosted thing and roughly once a year it does something irritating that costs me an evening. In exchange nobody can put six years of my notes behind a paywall. That trade is either obviously worth it to you or obviously not.

Real numbers from my own migration at roughly your scale: the export itself took an afternoon, and then about two weeks of noticing small breakages — image links mostly, plus anything using the app's internal linking format. Plan for the breakages, not for the export. The export almost always works. It's what the export silently drops that costs you the fortnight.

Whatever you choose, run the export once a quarter and actually open the output. Half the people with an export button have never once pressed it. Mine was quietly producing an almost-empty archive for four months and I only found out because I got curious.

Check what happens to attachments specifically. Text nearly always exports fine. Images and PDFs are where apps keep things in their own store and hand you a folder full of hashes with no filenames.