You are overthinking this, please don't stress about it. The way you're doing it is absolutely fine.
I had similar utter meltdowns about formula prep when DS2 was tiny so please don't think I'm being critical or patronising. It's a totally normal thing to stress about but your brain likes to overcomplicate things at this stage of parenthood.
Just remember it's not a magic spell - there is a logic behind each step and you can follow the logic directly rather than the steps if that works better.
The main reason they have the whole "Boil 1L of water then leave for exactly 30 mins" stuff is because it's a measurable way of knowing that the water is over 70C if you don't have a thermometer handy. The NHS has guidance that its advice must be in plain language and accessible to an average 9yo. This is to ensure as many people as possible can follow it accurately. It's basically deemed that reading a thermometer is more complicated than boiling a kettle - which TBF, even though reading a thermometer is a fairly simple task, boiling a kettle is even simpler. Plus I've never been in a British house that doesn't have a kettle, but not everyone has a cooking thermometer.
The only important thing about the temperature is that it's over 70C. That's the temperature recommended by NHS/WHO in order to sterilise the powder.
Some people get very worried about the having to leave it for exactly 30 mins but this is irrelevant as long as it's hotter than 70C, so just-boiled water is absolutely fine.
Some people claim that if you have the water too hot it is going to destroy nutrients in the formula. I think if this was true the NHS and WHO would include it in their guidance. They don't, so IMO you can ignore it unless you're using a brand where it actually does make it clump up (but they tend to recommend making it with 40C ish water IIRC). In order to avoid overwhelm around decisions like this, I made a point of only accepting NHS or other relevant information, particularly when it comes to risk factors, rather than trying to follow every possible thing everyone on the internet said because you will go mad. They all contradict each other and promise absolutely terrifying consequences. But the NHS advice is based on fairly robust evidence. If they're not telling you it's a risk, it's almost definitely not. Or if it is, it's MUCH smaller than the scary internet post makes it out to be.
The first piece of guidance is to always make up powdered infant formula with hot water.
The second piece of guidance is about making them up straight away if you can, so strictly, no to the "can you do 6 at once" question... but they do give guidance for how to store made up formula, if it's made up with 70C+ water, you can keep it for this amount of time:
2 hours at room temperature
After being cooled:
4 hours in a cool pack
24 hours in the back of a fridge
So technically you could make up bottles in advance. This is what we used to do when we were topping up. We'd make it about an hour in advance and leave it to cool on the countertop because we'd find after about an hour, it was drinkable temperature.
You can also use ready to feed cartons, partially used cartons can be kept according to the same guidance. We used to use a sharpie to write the date and time on the lid (they are bottles where we lived)
And again remember none of this is magic or mystery, the logic behind the combination of the hot water + sterilised feeding equipment + time limits is simply the limiting of bacterial growth.
If the formula or bottle is contaminated (which is rare) then the sterilisation/hot water should reduce any bacteria present to a tiny amount that even a newborn's stomach can process without problems, and the time limits ensure that even if there was a tiny amount of bacteria present it cannot multiply to a level that would make a baby ill.
Arguably the NHS/WHO advice is overcautious, because dry formula powder is not a conducive environment for bacterial growth, you need moisture for that, the risk of increased bacteria levels only really start once formula is made up. The babies at highest risk from formula contamination are those under 2 months old, so there is an argument that after a baby is over 2 months old, there ought to be logically no problem in feeding them formula made up with cold, lukewarm or room temperature water, as long as you're feeding it straight away rather than letting it sit around made up. This is what various other countries advise.
If you can find a way to make the hot water method work then it makes sense as a sort of belt and braces method. But hot water + feed immediately is likely overkill and probably running Swiss Cheese theory (if someone makes a mistake with one step, it will be caught by all the other steps).