[Google Pixel 6] flash713:
If I were you I'd maybe get some logs and go through them and see if you can find anything that way. Maybe add logs to your op here.

There aren't many ways to backup things nowadays for pixel 6 but what I have used to backup apps + app data that works well when restoring is Neo app from Fdroid store. As far as backing up goes, at this time I've used nothing that is/has been as solid as Neo. I miss the days of just being able to boot one of a few various custom recoveries, making a nandroid backup to a USB thumb drive with my OTG cable and restore it whenever it was needed but things change. Sounds like maybe something was left behind from your kernel install. If you really dislike formatting I'd stay away from whatever you installed here to get in this loop but definitely backup using Neo once you get going again and put your backups on thumb drive or you PC or somewhere other than your device so you can restore them when you need to. I don't flash nowhere near as much as I used to but I used to flash my device(s) many times a day , everyday for many years but I remember years ago on another device I looped and wiped my cache and system and reflashed rom and ended up booting up. But that was another device years ago. Maybe try adb sideloading instead of factory image flash without wiping data. Try that and after it's done installing reboot to others slot and sideload it again... These things probably won't work but you never know until you try. Did you have magisk installed before and not now? Maybe flash stock kernel patched with magisk to both slots if so. Hate to say it but you will probably end up having to format data but idk because I don't know exactly how you installed the kernel "in detail" or exactly what kernel it is or if anything else was changed from before you looped ... No logs...

I've been using Android since the beginning (G1) and I've been tinkering with my devices since back in the Nexus S days on a regular basis and I've found that the best way for to get the help I need is to try like hell to figure whatever it is out myself by searching for the problem I'm having, I mean like really searching, sometimes for hours , sometimes maybe searching for things related to the problem I'm having that show up in my searches, sometimes the answer can be in another devices forum, piecing together bits of info from various places to try and solve whatever it is... etc ... etc... But the best way to get help is do research on it, take some notes, try various things to fix it, then if I still can't figure it out make a post about it here and in op attach every log I can possibly gather from said device, include all the things I tried to fix it with myself "in detail..." My exact installation process, you know like: reboot bootloader, was on slot A, executed ./flash-all.sh for factory image xxxxx, rebooted system ... The more details you provide the more accurate people will be when answering and the more people you will get to reply.. I'm not trying to sound like some asshole or anything at all. Im just speaking what works best here to get answers. Sometimes you just don't get any answers even if you post 18 paragraphs with a chart and a line graph, things you tried, what all you tried it with and links to it all. 🤣 But anyone here who does this will most definitely stand a much better chance at getting some replies on whatever it is. I sometimes read over on Reddit and Tele groups , sometimes answers can be found that way. Sometimes people just give you the silent treatment.. Even if you do your homework. 🤣 That's just a thing with anything related to development that happens. I always used to think when I would get no replies that the other guys and gals who know the answer are thinking to themselves, "oh look at this idiot here!" "He clearly didn't graduate from Harvard top of his class with a PhD like we did. " " Such an imbecile!!! " But what could have been going on and more than likely was/is is life , kids, bills, sleep, relationships, covid19, and tons of other things..

Some of this development stuff is a real beech to get and everyone works hard to get it and many aren't too fond of just giving random people answers that took them many sleepless nights to get. But if and when a dev sees someone with a detailed post , things tried.... etc ... etc .. Theyre much more likely to throw some knowledge your way. That's been my experience anyway.