For any given volume of alcohol it would be advisable to maximise the amount of plant material used. With 50g MHRB and 300mL vodka, tannins precipitated over the course of several days at room temperature following the hot extraction.

Have you measured the temperature of your freezer? And did you keep the vodka in a tightly-sealed container while it froze? -22°C appears to be optimal for ice crystal separation, although this still needs confirmation with an actual thermometer since the thermostat on the (domestic) freezer has not been calibrated.

Vodka-strength alcohol has been found to be optimal for tannin removal via lime treatment. It also has the advantage of being cheap, readily available and food grade, at least in most places as regards the first two points. Recently, a brief test with running 69.5% ABV grain spirit through a filter funnel filled with the tannin-lime precipitate showed that a noticeable proportion of the tannin redissolves in the stronger alcohol. Thus, vodka-strength alcohol also brings efficiency savings regarding lime usage - more lime, while being relatively cheap, also becomes more tedious both to mix and to filter.