That's answering a pretty different question than the one asked though. If the GM's "job" is to ''balance for a more powerful party" with no say on how powerful is too powerful there must be tools built into 5ethat enable that gm to finesse monsters encounters & even individual PC strengths to the needs of their group. Past editions had various tools for that & I even named a few. You seem to be answering a very different & unasked question.I can't speak for Mort, but I can say that there are plenty of tools out there in the books, and on Youtube, and on Reddit, and on these forums, etc... Let me give you a suggestion for how to discover them yourself.
I suggest planning a one shot for your players. Tell them to build 5th level PCs using point buy, and that each one gets one rare item, two uncommon items and unlimited mundane (nonmagical) items. They also get three uncommon spell scrolls / potions of their choice. They can use any WotC class/race options.
Then, you get to build a one shot dungeon to challenge them for a five hour session. You get to use a small cavern and a road for your battlemaps, and the monsters you get to use in your build are:
19 goblins
4 wolves
1 bugbear
4 traps that have DCs no higher than 13 and deal no more than 2d8 damage each (but can hamper PCs, such as a snare or a pit)
If that sounds familiar, you may have played an intro adventure for 5E. These are appropriate challenges for a 1st level party with starting gear, and you're going to challenge heavily armed 5th level PCs.
Think about how you'd use your resources to make an interesing challenge for the PCs. Think about how (and why) the monsters might be prepared for the PCs and use their capabilities to provide challenges for the PCs. Figure out things the monsters might be doing that the PCs might have to disrupt. Experiment. Think outside the box - but make sure the story makes sense. If you try this a few times, and succeed in building fun one shots this way, it may influence how you approach the rest of your adventure building in a positive way.
If you need to point at YouTube reddit & these forums as examples of tools 5e provides the gm you've listed three tools that are explicitly not things that [k]5e provides°. To go on and even suggest specific monsters for a group of a specific level very much calls into question if 5e actually provides[j] any tools for the gm to finesse encounters & even specific PCs as older editions once did. Our of the three tools you listed that are not provided by 5e reddit was created in 2004, YouTube 2005, & it looks like these forums seem to go back to 99 putting them squarely in the time frame of multiple past editions predating 5e. If there are "plenty" provided by 5e itself, why can they not be named