iMac (Retina 5K, 27-inch, Late 2015) upgraded from High Sierra to Monterey 12.6, need help reverting back to High Sierra from Time Machine

Greetings, I have a wonderful iMac (Retina 5K, 27-inch, Late 2015) that ran just fine using High Sierra, but I made the rather foolish decision (in hindsight, as this iMac is literally the oldest model capable of running 12.6) to upgrade to the latest OS - and for many reasons I don't need to bore you with, I have ultimately decided I made a very bad decision. To summarize, In very broad and generalized terms,

1) it's just far too taxing on this hardware, and

2) All the applications I rely on are no longer compatible and purchasing so much new software is not a viable option

Now, if I had done this a couple days ago, I think it would be straightforward enough to just revert with a time machine backup, but regrettably, I was really giving 12.6 a thorough chance and weighing everything which took time, and dealing with a lot in my life at the same time greatly delayed that process even more, resulting in one of my big problems I am desperately hoping this wonderful community can tell me a solution for -

I DO NOT KNOW what date it was when I installed 12.6. I think there is a VERY good chance I have backups going back far enough to when I was running High Sierra, I just can't find out how I can tell from the backups which ones are post-OS upgrade and which ones are pre-OS upgrade...or if there IS a way to find this information from the backups.

This is hard for me, I worked for years in IT in Ivy League acadamia exclusively with Apple products, and was always my own expert, but a career change 15 years ago left me slowly losing all my expertise and suddenly I realize I know almost nothing anymore as I haven't kept up, and I suddenly realize I've fossilized, I'm almost useless and very lost! The little knowledge I still have is enough to add this -

I DO have the system DVDs that came with this imac. I also have multiple hard drives and time machine only backs up the drive with the OS, so I am pretty sure I have a handle on how to keep all my data, including I can easily backup anything created after the time machine backup with High Sierra, between just copying it to one of the other drives and I have an offsite online backup service as well, I use Backblaze. So I think no issues there.

I also feel pretty confident I could clean install an old OS, even prior to High Sierra, and upgrade back up to High Sierra if that's the only way, as long as there are complete system install disk images out there I can download, there always used to be I remember, so if that's my only option can that still be done?

Thank you ALL so much for taking the time to read this, and I can't tell you how much this once-all knowing Mac guru appreciates you current all knowing and all powerful PROS giving me your time and expertise, I'm so serious, this is the first time I've realized I'm totally helpless and lost on this one and I have a new appreciation for getting old and being clueless, haha. I hope there's an easy answer and easy fix!!! Because this old iMac just cannot do what I need it to it's got to get downgraded.

Thank you so so much for any help.