Do I really need to build the whole LOS? Shouldn't the kernel itself be enough? I already have the zImage with the driver enabled.CH341 is available on the kernel from what i've seen, i think it's just disabled to save on overhead and size so only changing the ramdisk won't help. Well, it should be relatively easy to compile a version of the kernel with it enabled, but you'll have to download the entirety of lineage's source to compile the kernel (even i find it a bit overkill, but it's what makes bluetooth work).
To download it and set things up, you'll need to follow this guide: https://wiki.lineageos.org/devices/h910/build#extract-proprietary-blobs
(pretty similar to other v20 models, i use h910 as a base since it's what i have).
Then follow the steps i provided on the kernel's github page to compile the kernel version you want depending on the branch you cloned once you have the lineage's tools and your device's proprietary blobs. To enable the driver it might be as simple as adding "CONFIG_USB_SERIAL_CH341=y" to kernelroot/arch/arm64/configs/msm-perf_defconfig, but i never used that driver, so i can't guarantee it'll work as easy as that.
I would try compiling a kernel with that driver enabled and send it here, but i'm currently busy porting a 4.4 kernel to the device on my free time and it will be a while until i have the time and disk space to setup lineage's tools here. If you have any problems, do share and i'll try to help.