are you still using sfml? i ask out of curiousity, not that it actually matters. sdl2? it is better supported.
i know you have no roadmap, and are not interested in libre-sourcing the project. i think that opening this whole thing up a little bit (not necessarily open sourcing it, but making the engine more extensible) would be great. i would love to see a more robust scripting system, and some sort of plugin/extension system.
i realize that the intention behind this is to make a better RGM. it has already achieved that, all that is left for that goal is bugfixing. i would love to see it one day grow beyond that, to a 2.5D-style engine that could potentially one day make games beyond wolf/doom. i envision rotational sprites (4 or 8 direction) as an extension, ai mods as an extension. a variable system already exists in the scripting, if we could gain a better understanding of the scripting system, if it could be expanded upon with some sort of extension system, all of this would be great. alot of cool stuff can be done with if/and/or logic and variables.
it is a fine piece of software already, and i for one have no issue with a price increase. it is easily worth 2 to 3 times it's current price in it's current state. lack of manual is the only thing right now that makes this software user-unfriendly and potentially offputting. have you considered creating a patreon page? i have already donated an extra 10, but you already spent that on vodka ;) my own economic situation is piss-poor here in the capitalist mecca of the usa but i would be willing to try and support the development of this software in any way i can.