As in most cases there are always more than one way to “skin a cat”. I will try to keep this updated with information on my progress.
This proved workable, but without a paid account on Formspree you cannot specify a return url.
Although this open source free project should fullfill my needs better than Formspree, as is it would required a cloud instance configured with a webserver and php. While that would be half the cost of Formspree, I would rather attempt to modify the project to support dokku.
I am trying this howto in my first conversion attempt.