This is exactly what these pugs are. Except they all obviously require you to join discord for voice comms and follow raid leader's calls. And they require that you have watched boss guides ahead, or you risk getting booted out for failing on trivial mechanics.
You can open LFG on week 1 of any HC raid, and you would see full pugs at many different progress levels. On Sun/Mon/Tue of the first BoD week, you could see at least 2-3 pugs at 8/9HC up throughout most of the day. They required 8/9HC experience to join (often forced to take 6/9HC players because of limited choice), and you better be ready to wipe an entire day before seeing a kill, not leave after 2 wipes.
There are many players who pug a full HC clear in the 1st lockout of
every tier. This is a massive time investment because it sometimes takes an entire day to kill a difficult boss (there are easy tiers: full Antorus clear in week 1 only took 1-2 evenings), and even the most dedicated players sometimes need to leave, so you keep replacing people and relearning some mechanics. But for the good pug players it's worth the effort to always stay ahead of the curve. If you pugged a raid in week 1 in gear below ilvl 390, you will never have problems finding a full clear pug starting from week 2.
You're much better off joining a guild, but whatever floats your boat. The advantage of the HC pug route is that it requires 0 commitment from you outside of a single progress week.
Here is one example of a
full pug group who killed Jaina HC on Sunday of the first week with average raid ilvl 387. The raid leader of this group (Oomdam) pugs curve on week 1 of every raid tier. If you check their roster for the first pull of the day and the last (22nd) pull of the day, only 7 of the initial 13 DPS players stayed till the kill. The rest had to go or were replaced. And here is their
first Jaina HC pull on Sun morning right after they killed Stormwall HC. Only 4 people from the first Jaina HC pull were on Jaina HC kill 8 hours later. I hope it puts to rest any suspicions that it's a well-organized group of friends rather than a full pug.
There are many pugs like that. Here is
one example. You can check how different their rosters were for first
Mekka and
Stormwall mythic kills.
There are two ways to go with such pugs. You can be a mythic raider with god experience, and play the revolving door game by joining random mythic pugs on random alts on weeks when you are bored. Or you can just be a skilled player and convince the pug leader that you're worth a shot. You join such pug at the beginning of a tier, and come back every single week to progress the next mythic bosses together.
Non-weekly (one-off) 6/9M mythic raid pugs for alts of mythic raiders are very common these days as well.