Hi and welcome to the forum!
You are correct, the LineageOS images contain partition images for system, boot, vendor, vbmeta, and dtbo. You can extract other partitions such as modem from stock OTA zips, provided you install the necessary tools and flash them yourself with fastboot. I do not believe this is harmful in any way, as the code on these partitions is suitably decoupled from the Android OS and partially runs on dedicated processors at runtime.
At the moment this may be an option to finally resolve the nasty random reboot during calls issue.
Please do backups of your data when you go from 16.0 to 17.1.