OffbrandGandalf

I backed Mike Shea's last "campaign" type book, and it was a great setting and collection of adventures. It was designed for levels 1 through 5 (similar to Lost Mines of Phandelver), but then at the back he included an appendix with ideas for, IIRC, a level 5-20 campaign. It wasn't super detailed, but it was enough to give DMs an idea what to do if their players really liked the setting and wanted to stay down there.

You know how you can read a campaign book and be like, "Why is this so confusing and hard to understand? Don't they care how hard the campaign is to run?!"

Well, Mike does care -- that's his whole shtick, so I'm definitely going to back this.

It's called Walker (1987). Directed by Alex Cox (Repo Man) and starring Ed Harris. Not currently streaming anywhere I know of, but if you can find it on DVD or something I highly recommend it. The less you know going in the better. :)

a worldbuilding game such as the quiet year, for the queen or microscope, hacked to set up a concise and thematic noir mystery inspired by fiction like Disco Elysium, The City and the City or The Nice Guys

That sounds amazing. I love the thought of a Nice Guys-inspired campaign setting.

Is the game you'd like to run also Lancer? That would be a wild coincidence.

Edit: Oops, nevermind, I just saw your other comment!

Electric Bastionland has been the RPG I most want to run ever since I bought it years ago.

After watching a surreal Acid Western (which started out deceptively straightforward and veered endearingly off the rails by the final act), I'm itching to run In The Light of the Setting Sun: Bonanza Edition. But I'm at the end of my current FIST: Ultra Edition campaign and don't want to rush anything. :D

Tricube Tales. It's the easiest game I've run, and strikes a perfect balance between rules lite narrative shenanigans and a traditional roleplaying game. Uses an easy to manage 1-3 pool of d6s, and all the GM has to do is pick a Trait (Agile/Brawny/Crafty) and assign a difficulty to the challenge (most of the time it's 5). Character creation is pretty freeform, with PCs getting Fate aspect-style Perks and Quirks. You can run just about anything in Tricube Tales. It's a real joy.

I'm also big into FIST ULTRA Edition, a game about paranormal mercenaries set during the cold war. Inspired by Metal Gear Solid and Doom Patrol, FIST is based on World of Dungeons, so it's a 2d6 system, sort of like PbtA with only a "defy danger" move. The heart and soul of the game are the 200+ Traits, which are a sort of combination half-class, feat, starting gear, and attribute score adjustment. When you make your mercenary, you get to pick two Traits, and the ensuing combination is always fun.

I am! It'll be an original game inspired by a few of my favorite movies. I have most of the text done and can't wait to test it out with some friends.

I’ve made a game called X’s in their Eyes and it’s a GMless competitive game about going onto a big TV talent show with the the aim of killing off one of the judges in a manufactured performance accident.

This sounds hilarious. :D

In my experience, the best way to prepare for running any RPG is to first have someone run the game for you. That can be hard, but the good news is, D&D 5E is the most widespread TTRPG in history, so finding a game shouldn't be too hard, especially since you're already talking online games. (Some areas are TTRPG deserts, but finding a one-shot 5E game online is as easy as it gets.)

If that doesn't work for you for some reason, I would suggest the following:

• Run a prewritten adventure, but don't worry about sticking too close to the book. You're already doing this, so good thinking! A published scenario (by that I mean, a short adventure, not a full campaign) does a ton of work for you. The one thing I would caution is trying to stick too close to the way the adventure unfolds. If things veer off in an unexpected direction, just go with it.
• Find a D&D 5E cheat sheet for you and your players. It's easy to forget a rule in the heat of the moment. Don't worry about getting "everything right" and stopping to look up a rule. If something isn't on the cheat sheet, just tell players you're going to make a ruling in the moment, move on, and look it up online later. Unless a character is literally on the verge of death, it probably doesn't matter anyway.
• Make a one page of prep notes for the adventure. Distill all the major NPCs, locations, treasures (if applicable) on a single sheet of paper. That way if someone asks, "Wait, who's the mayor again?" You won't have to dig through the adventure, you can glance down and say, "You mean Fuzwibble McShiddlebins, the Tabaxi male Cleric?" and watch their jaws drop and have everyone pronounce you Best DM Ever (this won't really happen, but it sure beats trying to find NPC info in the heat of the moment.)

Finally, I'd like to share a few thoughts about what it means to "DM effectively". Running a game is an art, not a science. Whole books have been written on the subject, and no two agree on most things.

The best thing to keep in mind is: everyone is there to have a good time. Yourself included! Players want to do cool things. The DM wants to thrill and excite and entertain their players. Mistakes are going to happen, but don't let them live rent-free in your head. It doesn't matter if anyone gets rules wrong, especially when you're all still learning. It's been years and I still can't remember certain 5E rules (passive perception and darkvision, yikes). My group still has a fun time playing, because the game runs smoothly enough. Things like safety tools, cheat sheets, one page prep notes, all this stuff is designed so you run into as few roadblocks to fun as possible at the (virtual) table.

Good luck, and feel free to ask any followup questions if need be. :)

P.S. I think this might have been posted in the wrong place? It probably belongs in c/dndnext, c/rpg or c/askgamemasters.

Yup, unified resolution mechanics tend to be easier for new players to grasp. (I suppose a good comparison would be if Call of Cthulhu had you bust out a d20 to resolve breaking and entering.)

Shadowdark (like White Box: FMAG) replaced the d100 thief skills table with a simple, "Roll advantage on Thief type ability checks." It's a personal preference thing, but I like it.

I have yet to run Shadowdark, but it seems like a perfect way to introduce D&D 5E players an oldschool dungeon craw campaign. There's familiar mechanics (dis/advantage), the old percentile Thief skill tables are gone, and DCC-style "roll to cast" seems like it'd be more palatable for folks used to attack Cantrips and ample spell slots. Plus, Kelsey Dionne is a fantastic adventure writer, so we're all but guaranteed years of top-shelf modules.