The idea is mapping on higher ROM any of the banks as we need them, and we can use that data with RAM from 0x4000 to 0xffff, leaving on lower ROM the bank 0 with our code.
'''Important''': when mapping data on high ROM with video RAM on 0xc000 you won't be able to draw masked sprites directly from that mapped ROM (reading video RAM is required to apply the mask). Either don't use 0xc000 for video memory or make a copy of those sprites to RAM from 0x4000 to 0xc000 0xbfff and draw them from there.
It is '''easy to support both CPC+ and Dandanator from a single code base''' using macros and generating both CPR and Dandanator ROMs is trivial. For example, these [https://github.com/reidrac/cpr-tools simple tools to make/dump CPC+ CPR cartridge files] can generate both formats (use RAW and padding flags to get a Dandanator ROM).