I’ll write a more detailed reply, but for the time being here are the scripts I used to create xubuntu for the pi https://1drv.ms/u/s!AvHY_kl4hMB4gQM56ORE2aveZ_CQ . Armhf and arm64 work pretty much the same apart from dtb files are in a Broadcom folder on arm64. Feel free to use the preseed files I used in xubuntu. With them you have everything to create a U-mate installer. The scripts I included in the zip to create an image from an OEM install are a bit hackish, probably best to ignore them.