So I went over the thread, went back, went back some more and I have the general recipe for modifying the firmware figured out:
1 unzip ksx file (7zip, izarc
https://www.izarc.org/, it's a cab )
2 copy nk.bin.comp to other directory
3 bincompress /D nk.bin.comp nk.bin_tobepatched
4 patch nk.bin_tobepatched
5 bincompress /C nk.bin_tobepatched nk.bin.comp
6 md5sum nk.bin.comp
7 modify install.xml
45c45
< <file checksum="originalChecksum">nk.bin.comp</file>
---
> <file checksum="newChecksumFromStep6">nk.bin.comp</file>
78a79,95
> <action>continue</action>
> </onFailure>
> </installStep>
> <installStep>
> <command>\windows\cmd.exe /c ren \Secure\Startup\infiniivision.lnk infiniivision.lnk.original</command>
> <onFailure>
> <filePath>\usb\errorLog.txt</filePath>
> <message>No factory lnk to move away.</message>
> <action>continue</action>
> </onFailure>
> </installStep>
> <installStep>
> <command>\windows\cmd.exe /c copy \usb\infiniivision.lnk \Secure\Startup\infiniivision.lnk</command>
> <onFailure>
> <filePath>\usb\errorLog.txt</filePath>
> <message>lnk copy from USB failed.</message>
> <!-- continue, without new valid link file -->
8 repack directory to cab file with
https://www.izarc.org/9 rename cab to ksx
10 copy to memory stick
11 add infiniivision.lnk to root of memory stick with
90#"\Program Files\infiniiVision\infiniivisionLauncher.exe" -l D3000BDLA -l SCPIPS -l WAVEGEN
update firmware
Boom!
Now I still have to work out how to find the exact locations for the patch (and how to get the checksum.
A long time ago I got the version 7.50 for the 3000T, patched and there the locations are
0x0DD19FB 99 69 -> FE 67
0x127929F 04 00 A0 E1 -> 00 00 A0 A3
0x12F4C9B B4 F1 93 E5 -> 01 00 A0 E3
I tried my hardest to find over what "area" the checksum is taken, why exactly _this_ address needs to change from 04 to 00, and why the third edit is there.
Unfortunately, I haven't come across the "understanding" yet, I tried importing the file into ghidra, but arm v8 little endian 32 seems not to be the expected architecture, ghidra complains a lot about there not being instructions... so even a hint on a correct set of paramenters would help. Binwalk gave me a long list of "file consists of html headers, pictures, program code" and if I let it split up the file (of the un- and the patched 7.50) , I found one file where the md5 differed and in there were the modifications, but I am not sucessful analyzing that file, either.
So some hint on how to understand step 4 would be appreciated, so I can pull it off on the newer firmware

Thank you very much everyone that put in the work! I