Hi Polikuo! I've got a suggestion then ...
If we followed how Porteus, Gparted Live, and how SystemRescueCd does it, there is no need for tc-install at all! For those three systems (so far), all that is really needed is simple file-copying after either unarchiving, or mounting an iso. (see the off-topic threads)
I have noticed some differences between TC and dCore - perhaps this might help. From my utility log:
TinyCorePure64.iso:
ISO analysis:
Image is an ISO9660 image
Will use '/boot/isolinux/isolinux.cfg' for Syslinux
Detected Syslinux version: 4.05/0x4f92e181 (from '/boot/isolinux/isolinux.bin')
Detected EFI bootloader(s) (from '/EFI/BOOT/efiboot.img'):
● 'bootx64.efi'
Disk image analysis:
Image has an unknown Master Boot Record
Image is a bootable disk image
ISO label: 'TinyCorePure64'
Size: 27.7 MB (Projected)
Note: File on disk is larger than reported ISO size by 888 KB...
Uses: Syslinux/Isolinux v4.05
Uses: EFI
Using image: TinyCorePure64-12.0.iso (29 MB)
And for dCore64:
ISO analysis:
Image is an ISO9660 image
Will use '/isolinux/isolinux.cfg' for Syslinux
Detected Syslinux version: 6.04/20191223 (from '/isolinux/isolinux.bin')
Detected EFI bootloader(s) (from '/EFI/BOOT/efi.img'):
● 'bootx64.efi'
Disk image analysis:
Image has an unknown Master Boot Record
Image is a bootable disk image
ISO label: 'ISOIMAGE'
Size: 202.6 MB (Projected)
Uses: Syslinux/Isolinux v6.04
Uses: EFI
Using image: dCorePlus-stretch64.iso (204 MB)
So the first thing I see is that a different version of Syslinux/Isolinux is used.
AND, possibly TC's file-size being off by only 888kb. Maybe enough to matter?
Which makes me wonder if TC's efiboot.img might have missing / corrupted components. Something awry in the El-Torito build chain? What's causing that 888kb discrepency and does it even matter?