If it were me (but is isn't) I would suck all the water I could get out of the tank, push all the gravel to one side, add top soil to the other side (empty side) push all the gravel on top of the soil side, add soil to that now empty side and than spread all the gravel around, and fill it back up

it could be a bit messy, but IMO if you fool around with mud/ice cubes and soil packets you will end up tearing it all down and have to start from scratch, in
which case you may as well do it now and be done with it, less work in the long run,
And I believe you will need more lighting as well, according to my calculations 60 watts of light over a 50gal tank only comes to 1.2wpg, and although Diana Walstad states 1.5wpg will work IMO 2wpg is ideal and will grow most plants including alot of the red ones
In any case I wish you all the best in your endeavour!