How do you "slam them prone" without some significant investment? Knocking people prone isn't something you can do without a feat, a special effect (e.g. wild shape), or a spell, as far as I was aware.
You can shove someone no more than one size larger than you as an attack, and roll your Strength (Athletics) vs their Strength (Athletics) or Dexterity (Acrobatics). If you win, you either knock the target prone or 5 ft away from you. PHB page 195-196.
Battlemasters and various monsters get special attacks that let them deal damage and knock opponents prone, but anyone can shove.
Edit: and regarding stats, there are very few monsters with Strength bonuses of more than +5 (particularly ones that would be likely to run off and get help), and almost no monsters with Athletics proficiency. High Dexterity and/or Acrobatics proficiency is even more rare. A mid-level fighter can easily have an Athletics bonus of +7 to +9 - or a rogue or lore bard could have something similar via Expertise without focusing unduly much on Strength. That gives you pretty good odds of success. Bounded accuracy is definitely working in the PCs' favor here (unlike 3e where using combat maneuvers on non-humanoid foes were pretty much doomed to failure because of size, Strength, and possibly extra legs and stuff).
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