Curious about that. In my game we got to level 5 and the wizard and druid were shut down to just a few spells that had annoying riders to slow the enemy down. Many spells simply did nothing at all. The math gave those spells about 10-20% chance. Also, Druid had intimidate which never worked for them so they gave up trying. Similar math. Just felt like spam cantrips and hope nobody gets crit down'd which was common since the enemy had very good chance of scoring crits on a regular basis.
At 5th, I think my main "spam" debuff was
fear. I can't recall if I had gotten
slow yet, but it's really good against individually powerful monsters. Giving up two of my actions for
slow that will cost the enemy one of their actions is a good trade, considering I have about three allies who also get three actions each per round. Level 5-6 is also rough for casters, because you're really feeling your proficiency lagging (martials get weapon proficiency increase at 5th and casters get casting proficiency increase at level 7).
This is, however, one area where spontaneous casting is better than prepared casting. If the situation calls for it, I can cast
slow round after round, but if I were a wizard I'd probably only prep one. And if we're fighting mooks, well, that's when the
fireballs start flying (well,
waterballs in my case. And now at higher levels,
cones of cold.).
Another meta thing I've noticed is that when you're fighting "bosses", it's fairly common that they're fiends. Fiends often have magic resistance, giving them +2 or so to their already beefy saves, so even spells with decent effects on a successful save often fizzle. But do you know what's good against fiends and undead? Holy water. While holy water only deals 1d6 damage, it also triggers weaknesses against Good damage in fiends which can be substantial. And even a miss deals splash damage.