I would love to see a special font module in Affinity designer that allows you to assign a object to a letter, for example you could ink the letter 'F' and then assign it to capital 'F' to a new or existing letters collection.
Then in any affinity document you can select the font to instead source from the collections and the selected text will be converted to collections font that you created. It will operate just like a normal font minus that any modifications to to the original selected collections file font will be displayed as a live preview.
It could work very similar to how swatches works with collections and you would be able to take the collection font file and work in it on another computer or device using affinity.
And finally when you are happy with the font that you can export directly into a TTF OTF or Webfont.
I have included a rough basic concept idea for the on display information and selection of letter by letter collections handle.