Might not be what you are after but I recently got the Fiio retro nano bluetooth. It is a small battery powered DAC/Amp, so you plug some wired IEMs into it, pair it with your phone and you have wireless IEMs. The reason I mention it is because the battery life is 7.5 hours, and the battery is user replaceable – standard lipo.
If your IEMs break, you just replace them. It supports lots of bluetooth protocols and can in theory transmit lossless.
If you insist on TWS, the battery will always be an issue, which is why the Fiio product appealed to me so much.