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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
I know umap and use it too, but unfortunately there is no api to load the data onto another device. I developed this app: https://github.com/strukturart/o.map and I have and use the openstreetmap.org api to load gpx tracks, if there were any markers it would be perfect for e.g. plan at home and use the data on the go.
A user can set their 'home location' in settings, which can be read. Furthermore, if logged in, one can set some 'preferences', a key-value pair with at most 255 characters (see the wiki for all information). Use OAuth to log in (for javascript, this library makes it easy). It is meant for editors to save some settings, but, well, I've been (ab)using those a bit too.