and, lol i thought it was some translation error.. no i wouldn't cut the bucket down.. if you have some air currents, it'll be fine. remove any leaves pointing down into substrate..
if you really wanted to, once it's taller you can add an inch or two of substrate if you wish.. i wouldn't bother though. When you transplant, before you bury anything, place the put into the hole you dug... is it at the right level? if not dig some more or fill it in a bit to get it to the right level. i try to get a layer on top of the transplant. if you ahd good watering habits, you shouldn't have any superficial roots, so removing a 1/2" off the top of a transplant isn't too big of a deal either -- one more tool to level it out the way you want.
When i transplant a larger plant in a 1gallon (vs seedling pot), i dig a partial hole with a slgihtyl udnerfilled pot. this way i can level it out and work the substrate around it to hold it steady exactly where i want it - e.g. if tilted i gently lift it and push some substrate under or pack in one side to hold it more vertically. Once sufficiently positions, then pour in some from sides to fill in the rest... hold the leaves up with one hand and dump it in, then level it out. Jst work your way around. this part doesn't take much ... dump and level. I do one half then the other. Make sure if anything is not even it is concave and not convest.. between the two options - water you put on top rolling (gravity/slope) toward center is better than toward the edges. It makes watering more of a pain to ensure no dry pocklets if ther's a slope or uneven top... the low side will sink in faster / gravity will pull it that way as you water the top... i have a sloped platform.. i slightly favor the high side with my drippers for this reason.