If you're running at 74C all-core you are thermal throttling, yes.
It's not a hard-throttle, but Zen2 boost algorithms start to pull back clocks over 70C. If you want to push your CPU further you probably need to spend money on either a very beefy air cooler, or water would be easier.
Let's say you invest in a beefy 360mm AIO and can keep temperatures under 65C, 6 cores all at max voltage and 4.2+ GHz is going to draw a lot of current. Your motherboard is okay. All MicroATX B450 boards have pretty mediocre/bad VRMs but the Mortar Max is one of the better ones. I think it probably could handle the ~175W and ~160A that you'd need to run your Ryzen5 3600 at the 4.4-4.5GHz silicon limit at ~1.4V
So yeah, interesting experiment, but you'd have to spend maybe $100 on cooling and realistically you could just sell your 3600 on ebay and pick up a 3900X for an extra $100 if you really wanted better cinebench scores. No amount of overclocking will turn a 3600 into a 3900X