Thank you!
An infinite mode is definitely coming. The current setup was made to implement a stopping point of "Wow, I beat this game! Time to move to the next one." or a reward of a good "spawn-when-you-need-it" card, but also works this way because enemies are bound to a level range (some spawn at the beginning but not at the end).
And they are getting stronger, just gotta make the sequence of enemies just loop every "final boss" kill.