I am not sure how having a non EFI graphics card effects choosing the startup disk from BootCamp installed in Windows. This answer is based on the assumption there would be no effect.
Older versions of Boot Camp can not detect APFS volumes. This problem may solved by using Apple Software Update to download and install a newer version of the Boot Camp Software. However, your Mac is probably to old for this to occur.
If Sierra is installed on a HDD, then an upgrade to High Sierra should result in no conversion to APFS. In this case, the Boot Camp software installed in Windows should be able to make High Sierra the startup disk, just as it now does in Sierra.
However, a conversion to APFS should occur, if Sierra is installed on a SSD. If High Sierra ends up booting from a APFS container, then an alternative would be to create a small (200 MB) jhfs+ formatting volume containing the rEFInd Boot Manager. You can configure rEFInd to silently boot High Sierra. BootCamp installed in Windows will detect this volume with rEFInd as macOS. If set by Boot Camp as the startup disk, then rEFInd will instruct the firmware to boot High Sierra. You need to properly configure the
refind.conf file for this to happen.
rEFInd can also be configured to boot other operating systems. However, to choose the default, you will not be able to use Boot Camp. Instead a script can be installed in Windows which will instruct rEFInd which is the startup disk. An example of the execution of one such script is shown below.