Don't worry too much about the Chinese/etc. IEMs and MMCX as for the most part they will be standard.

Usually it's just Sennheiser, Etymotics, etc. that have those weird proprietary/altered versions of MMCX and so it's MMCX but not really as you have to get the MMCX version specific to the Sennheiser/Etymotics/etc. IEM, lol.

It just sucks because you have to spend more money on new cables or adapters/converters. Alternatively you can also try physically modifying the ports/plugs or cable of the IEM but that voids warranty and so not worth if those adapters/converters are easily available, like these days you can often fine them on Amazon but with just a bit of price increase compared to AliExpress/China.

For the Sennheiser/Etymotics/etc. the MMCX ports will be loose and so it will immediately fall off. Like you can tape it and it'll maybe still work (I did that with Etymotics and UTWS3/5 for a while despite the bad angle, just for testing, lol) but ya not a good idea for long term use.