Hey @DamDave hope all is well that side!
I just recently took my bin apart, but for the longest time I had an old compost tumbler standing here. I would dump all my spent FF soil in there with a bag of elemental blend, give it a few spins and done. Then I unload it all into a big 100L pot to stand and cook till I am ready to use it.
I always test ph and ppm before using, but in your case having to mix quite a couple of big heaps right now, a setup like this will save you a ton of back ache and if you anything like me you would do anything to get around those back aches.
anyway, I know you mix once off and done, but these things really are great for making compost too, you can literally do everything inside the bin and leave it be and just rotate it couple times once every week. so it's not like there's no other uses fpr this thing.
here's a few examples. first one is more permanent
second one is without a pole / A frame configuration and instead have the barrel rolling on wheels, with this configurarion I would just advise to avoid swivel wheels otherwise you gona have some trouble. fix wheels would work better than these trolly wheels.
last one is probably the most practical and most sensible, you could do the same thing, but lay the bin horisontally to reduce impact when you roll the bin. what happens a lot with these vertical bin systems is people flipping them too fast and having the entire weight of the medium inside fall to the bottom either damaging the bin or structure. horisontal bin gives for a more even roll and spread inside the bin. just have to roll it slowly.
small wooden frame. A frame would work nice aswell. a pole. and a bin.
nice little backyard project that can save you tons of time and effort in the long run. if you a guy that regularly mixes soil or wana make compost