I realised my goals had been to go to the gym a lot and feel tired.
+1 here. The old "you're not getting better unless it's always hard AF" mentality..... It does not work.
repeated training to failure (squat ‘til you puke type stuff). The kind of training you might get away with lifting three days per week but will stuff you if you’re lifting six (without the benefit of PEDs).
Also +1, big time. I would add that even at three days a week you might end up with injuries. Three days a week, to "failure" might be doable for a couple weeks....but I personally wouldn't do it past that.
It's an interesting thing; if you go to 100% one day, are you really training at the same level of "100%" the next session? I mean, maybe if it's a full week after, but in the same week, I would think that it's impossible to actually max out one day, and then repeat that level of strength/intensity/whatever even two days later. If you did, then I would say you didn't actually max out the first day.
I write that to hopefully make the point that I think a lot of people overestimate their recovery ability. Paraphrasing something I saw from Gymnastic Bodies a while back, "you can always do more tomorrow, but you can't undo what you do today."