I agree with you, grado benefitted with more power, btr5 or shanling up4 (same core components, different tuning) as a middle ground will serve you good.I have considered the BTR3K, but it only puts out 50mw at 32ohms, which is less than the 80mw from the Fiio M6.The M6 drove my Grados OK, but I wouldn’t really want less power.
Plus, compactness and portability isn‘t a big priority for me. It would mainly be for static use, but needing a bit more flexibility than a desktop unit. This is why I’m looking closely at the BTR7.
Anyway regarding bluetooth connectivty, ive never tried any bluetooth tws/amp that can really connect from different room (yes including btr7), so dont expect too much and never go too far from your phone