yeah, there is a small chance that the ending will be anime only, but from the sounds of it the writer has mostly stopped releasing light novels right now because he has been working closely with the anime company to make this (and wants all the future stuff eventually animated as well)
given the next 3 arcs are already written and posted online after this wraps up he can likely go back and focus on the writing and get the next few arcs out in the next 1-2 years, but yeah it's gonna be at least that long before we get more re:zero anime.
Ah, a collaboration of sorts, I see. Well, if the next 3 arcs are done, then technically they should have enough material for another season next year, hopefully?