I'm a heavy EQ user, and from what I've read, ES100's EQ application is way better. The EQ function on FiiO works but it isn't very accurate. Just turning it on in the app colors the sound with all the sliders set flat. It's certainly not parametric, either.
I don't notice any lag on my laptop when connected via LDAC, but on my old android phone with BT 4.2, there is maybe 25-75ms when connected via SBC. Tolerable for most stuff on YT.
Other than those two small things, I absolutely adore the BTR3k and would quickly buy another if something happened to it. Be aware that the DAC maxes out at 16 bit and 44 kHz. BTR5 is 24bit, but it has ESS DAC whereas the BTR3k has AKMs. It sounds absolutely fantastic, and I can't tell a difference from 24bit and redbook audio, anyway.
I had been set to buy either an ES100 or BTR5 as my primary device but I ended up liking the sound on this thing so much that I just didn't bother. fwiw.