Believe me, I’m overly familiar with the Magic users who hurl fireball on the outset of every encounter. I just wrapped a campaign where the party’s sorcerer would run out of spell slots by at least the second battle of the day, if not the first from all of the fireballs he would cast. I challenged the party’s lack of resource management by interrupting rests whenever they took one because they were out of spell slots vs it was in-game time to take one.

I don’t know what to tell you OP, if this is the case. It sounds like you already spoke with the DM and he is set in his ways of not really challenging the party. Maybe if the other short rest resource players spoke with him, he might change, but it’s doubtful.