Blockquote: I would like to follow the MISRA C++:2023 coding guideline[s]. But it targets C++17, while the code and compiler follow C++14.

Do you want to run your diesel engine on petrol?

Blockquote: But I would see C++14 as a subset of C++17 (with only few breaking changes). Would I not be MISRA-C++-2023 compliant or only with additional rules?

Use Autosar C++ 14 Guidelines instead, which is a 70% reference to MISRA C++ 2008. You will not be able to make C++14 compliant with Rule 4.1.1 without a Deviation. Further, read the MISRA Compliance:2020, which is mandated for use with MISRA C++ 2023.