Will definitely try this to compare to MessagEase. It's pretty close to something I've been planning to do using Keyboard Designer (Google Play link). So the left thumb mostly has all the vowels, what was your rationale for choosing the consonants that went on the A and E keys?
Maybe assigning some slides to syllables ('ing') or even frequent words like 'the' would be useful? It really sucks that you can't slide on space, though. Let's look into Keyboard Designer - maybe it will be a step up from abandoned Multiling O...
The problem you've got is that the keyboard (qmk) is only sending keycodes, the translation into the symbol to display happens in the OS.
I can only think of a couple of options:
set your keyboard in the OS to a UK keyboard or some other country that has £ and €. The US International layout might also be possible.
since you're using Windows you don't have Compose built in, but you could look at http://wincompose.info/
Old thread, I now, but it's available for download if anyone else stumbles in here. https://www.microsoft.com/en-us/download/details.aspx?id=102134
If you want some ideas, here is another customisable keyboard. There are assorted presets to explore.
It is more difficult to configure... requires a text file config.
But you can do things like this, my Z8 designed for swiping.
MessagEase is great, but it doesn't seem to be updated any longer.
Keyboard Designer is another great option that's still actively supported.
I have something similar to this but if I could do it again I'd probably spring for something more like this one.
I think having more pedals would not only be obviously more functional but the triple layout would give it a much more stable base and be a lot easier to maneuver around under your desk with your feet. The single pedal I have is cheap and it very much feels like it, I would just put down a lil more cash up front if you think you're actually gonna use it!