My favorite would be an iode rom for this device, but iode only got oneplus 11 salami on the website. So I thought I might help getting this accomplished.
Steps so far:
I found an alternative lineage-OS based Android 15 rom already which works. And I have dumped the payload with payload dumper go. I have not yet installed a modified bootloader with magisk.
If anyone could link me to an article which explains the next steps in particular for iode, i would really appreciate this. Best, Tobi
Here are the official iodé build instructions (basically lightly modified LineageOS instructions): os / public / manifests / android · GitLab
@petefoth and I build using the LineageOS for microG docker environment: Home · lineageos4microg/docker-lineage-cicd Wiki · GitHub
You can also make a request here: Request unofficial builds for unsupported devices but the sources for the unofficial LineageOS build you found are a key requirement,
There is also of course the GSI image you could use.
Full disclosure: until now I have not personally made builds for any devices besides those officially supported by LineageOS (but not officially supported by Lineage), so others will probably need to give a bit more guidance.
To get any further, you will need access to the source code for that ROM.
The tricky part will be getting the device-specific binary blobs. For devices officially supported by LineageOS, these blobs can be found in TheMuppets GitHub or GitLab repos. If you are lucky, the person who made the ROM may have made the blobs for this device available in some repo. More likely, you will have to extract them for yourself from the ROM zip file using the instructions from the LineageOS wiki here and here. Sadly, when I tried to do that, I was never able to make that work 
. All the devices I build for either have binary blobs in TheMuppets or the maintainer of the unofficial ROM I use as my base have extracted them and made them available in their GitHub repos.
Also, to save yourself some time, try building for a device that is supported by LineageOS, which will confirm that your build setup is correct. Only then try moving on to building for the device you care about.
Tha’ts about all the help I can offer. Good luck!
Dear @Rik, dear @Petefoth: Thanks you for your extensive comments. These are very valuable for me and I will read through the guides you have linked.
As of now, I only got access to the img (boot, vendor_boot) and recovery and and the zipped bin (payload.bin) files. Recouping the particular 47 img files with payload dumper got me towards these, but I agree: Compiling from source for a known device with the source files,( and the blobs as far are available for the vendor specific hw) would be the best choice.
Unfortunately do not have any hint for the person who have compiled them other than his “name” Saskhana on sourceforge. I was thinking to sign up to the telegram channel to find out more, in between kyrillic and chinese posts.
But first I will follow your guides, so thanks again. one last question. From the ubuntu build instructions I saw theat they are recommending for a custom recovery with TWRP - which is not available from TWRPs website for the UDON and not even any OP11 device in question. Would you strongly recommend for it before any further action? Or would an Magisk patched bootloader be enough here?
Many thanks, Tobi
OK I did a bit more digging. I could not find any custom ROM available for this device. The nearest I could find is this thread in XDA Forums. That has links to the GitHub repos owned by ther person who claimed (in April 2023!) to be
working on source for custom rom development stuff.
The last commit in any of those github repos was in May 2023. So it looks like they didn’t get very far. Though thye did create a repo claiming to contain the vendor binary blobs
So, to get any custom ROM working for this device, you will be pretty much starting from scratch. To get IodeOS to work, you would need to get LineageOS built and working for the device, again starting from scratch. From what you have written I don’t think you are in a position to do this. No advice that @rik or can give you is going to get you to a point where you are able to make a custom ROM from scratch.
The Lineage OS Wiki FAQs have a question which covers this
Where are the guides so I can port LineageOS to my unsupported device?
There are none.
See what other similar devices did to get them to work.
So, I’m sorry to be discouraging, but I can’t be any more help, and I have no more to contribute to this thread.