What is the opinion of Reddit about the
FiiO RC-BT HiFi Wireless Bluetooth Lightweight Headphone/Earphone Replacement MMCX Cable with aptX/AAC/SBC Support and Mic?

I don’t understand. You say you’re ok with a neckband, but you say you don’t want what I recommend with a neckband? All IEMs made for sound quality don’t come with a Bluetooth cable (or as a complete package). I don’t know what you mean with “not as comfortable”. All you would need to do is remove the stock cable and replace it with something like this
or this. If you can stretch your budget, then I’d recommend the FiiO FH5…with a replacement Bluetooth cable for a total of around $280-$300. What I outline above will give you literally everything you want and meets all of your requirements; I have to assume that there is some form of misunderstanding. There is not a “complete package” at any price point better than these.

I own one with no issues

https://www.amazon.com/dp/B07D8J8CJ9/ref=cm_sw_r_cp_apa_i_gyDCDbB2PYMDD

The cable is detachable on a lot of IEMs

Edit: you would replace the cables with something like this https://www.amazon.com/dp/B07D8J8CJ9/ref=cm_sw_r_cp_apa_i_HbaGCb6SZZGK8