Ok - so a lot of (important) questions to unpick here.
1) The only version of TWRP for the FiiO is 2.8.5.0. I haven't had time to create a new one, frankly because there isn't a real need for it. TWRP 3.x will improve things for the custom ROM but won't be good for backward compatibility. Eg: If I had to implement F2FS support for the FiiO, going back to an older custom or vanilla kernel would make the partitions unreadable.
2) When I originally bought this player, I had also asked for viper4android systemwide support (ie via magisk/SuperSU) - but it's not possible without some significant modifications to the system audio configuration; which is arguably not worth the while. Additionally - and perhaps more pertinently - V4A internally uses only lower-bit transformations, which significantly distorts the audio envelope. If you're just using 128kbit MP3s that's fine; but FiiO have actually done quite a good job in preserving the audio fidelity across the hardware and software stack; so arguably it's a shame to downmix it through V4A. I'd suggest you play around with Neutron as you can get very similar effects with some of the filters. If it's just FiiO Music Player you want then use an older version of FiiO Music which still supports (paid) V4A plugins.
3) Bluetooth support on the X5iii is through apt (not apt-HD) codec; which means you're limited to 48kHz. It's not bad (my Sennheiser and Sony sound great on wireless) but you won't get 192/384kHz as you do with wired.