You asked for ideas on other free GPS tracking apps for Android that we might like you to support then please. I have used OsmAnd very successfully for both planning and tracking boats along rivers and canals. It might be a useful app for you.
I'd have to see if it supports making external API calls - if it can't there's no easy way of integrating it.
OSMAnd does support making external API calls.
It has an editable URL template, the default example being
Quotehttp://example.com?lat={0}&lon={1}×tamp={2}&hdop={3}&altitude={4}&speed={5}
and when enabled makes a request every N (default 5, configurable) minutes.
Parameter {6} is the bearing but for some reason this isn't in the example URL.
We're basically building a PWA to do tracking and other things. Not sure when it will be ready for testing at the moment due to work and other commitments.