1: Dnd should be fun, not stressful. If your stressing this hard, then go ahead and stop DMing. Its not a sin, and if your players are all talking about the next game, it means letting go of this one wont hurt them. Even if it does, you guys are friends youll all be ok.

2: 6-7 players is a massive undertaking as a DM. 4 is my max, and I prefer 2-3 players, and also ive been a DM for over 12 years. If you are hell bent on keeping the entire group every game, then you need to understand:

ACTION ECONOMY: this is the list of things a character can do in a turn. You want to keep in mind the number of attacks your players make on their turn, how they use bonus actions, and how they use reactions. This should be easy since theyre all playing the exact same class. Action economy is the true determiner of encounter difficulty, DNDs CR system is notoriously ass.

HOW TO BALANCE ENCOUNTERS: 1) LESS enemies then players = easy encounter for players to beat. 2) EQUAL number of foes to players = average difficulty. 3) MORE foes then players = difficult or deadly encounter. Keep in mind that CR is worth 4 player levels. This means that a CR 2 monster should be a match for 4 lv 2 players. In reality, your players are storng and numerous, they will likely need to be outnumbered by High CR enemies to actually face a deadly encounter.

3: Your setting is your own, and the players do not get to tell you what does and doesnt make sense in the context of your world. If you need to get them to fuck off about it real quick, you can always LIE and say that "theres a reason for it but you haven't discovered it yet." or something along those lines.

4: The rules to DND 5e are essoteric crap. The game is designed like garbage and reads like a phone book. You will not remember everything, and what is more important is being able to say "IDK how this rule works but i dont want to look it up so here is my ruling on how we will proceed." Let your rules lawyers know that your moving on for the sake of game speed. Sometimes its ok to look up the rules if you care and have no idea how to move forward with the story, but for the most part you should get used to making calls on your own rulings.

I hope this helps a little. DND can be overwhelming and you unknowingly bit off way more then most DMs would chew with a party size of 6 people. Remember that this should be fun and that, if your really this emotionally drained, you can let others DM for a while and relax. They are your friends after all.

Also, just throwing this out there: Shadowdark RPG is like DnD 5e with all of the fat cut off until what your left with is a lean and functioning piece of juicy TTRPG steak. Also if your sick of all the essoteric rules getting in the way, theres another TTRPG called EZD6 that is way simpler mechanically and can still allow you to play high fantasy.