Well, as far as I can see the respawning works as expected. So, nice job on that!
As for the Enemy+ on timers, that would be nice, but I don't see it as a high priority- the respawn as it is covers 90% of the use cases.
Timers would be useful if you can use them for both first spawning AND respawning, but even then these mechanics(spawning/respawning) are better served tied to a variable, similar to how you can unlock a number of chests or disable traps with one variable. In my experience trying to time when to do the respawn is much harder, then just tying it to a sub boss death(for example). And spawning would allow for putting more powerful enemies at play when you have cleaned a certain progress point in the game.
The only serious plus of timers(compared to variables) is they would open the possibility to make arcade style games, if they allow for short respawn times.
And thanks for the nice words- they are much appreciated :) I don't mind helping out with some advice but you're getting close to the point where you'd either have to rework core mechanics like combat(which work good enough) or start adding things outside the scope of a small engine.
But hey, if you decide to make a Tiny Visual Novel Studio or Tiny Arcade Studio engine I'll be happy to help on those too :)