Freeze your cactus thoroughly before processing it any further. This helps to rupture the plant cells, making the extractables more accessible.
Use distilled water if you live in a hard water area otherwise you'll end up with tons of sediment. You'll still get a fair amount of sediment anyhow. You will not need to add much acid at all - the alkaloids are present along with soluble organic acids in the cactus, otherwise the plant would be impossibly basic. Adding a little ascorbic acid or citric acid seems to do no harm.
With 10kg cactus, reducing down tea without caramelising it will leave you with about 10L of liquid. Your starting volume will be in the order of 50- 100L. That's a lot of liquid to be dealing with. You might want to try using something like 2kg before embarking on an endeavour of which you have apparently very little experience.
Be prepared to spend A LOT of time on the boil - you don't want to burn your pans by hurrying.
If you are willing to stand beside your pans the whole time, there is a technique for indoor boiling down of the tea that avoids producing huge amounts of steam, which can be an issue in areas with cold/damp winters. By having three sets of lids for each pan it is possible to use the lids as erstwhile condensers.
After allowing a maximum amount of condensation to build up before dripping occurs, the water can be wiped off the lid (lid 1) with a silicone spatula and saved for later use - it is distilled water, after all. Meanwhile, a previously water-cooled lid (lid 2) has been placed on the pan already. The freshly wiped but still hot lid (lid 1) is placed, preferably floated, on cold water. The third lid (lid 3, of course) has been removed from the cold water and dried so that it will be ready to place on the pan when the next lid (lid 2) is removed, wiped dry and placed on the cooling water. The cycle continues as long as necessary.
Why go to all this trouble? One benefit this procedure offers is prevention of a crusty build-up of drying cactus goo which slowly creeps up the side of the pan when evaporating without a lid. This goo also forms a thin crust on top of the cactus soup which slows evaporation. IME, the forced condensation onto a cold surface sped up the volume reduction quite noticeably, as well as greatly reducing the amount of steam escaping into the kitchen. If you have the time to commit, I would suggest giving it a try. It may be a vible domestic alternative to pilot-scale evaporation equipment you'd need otherwise.
Another alternative might be to fit a suitable condensing apparatus (still-head and condenser) onto a saucepan lid, but I'm not describing that here. A modicum of research one's own would be of benefit to anyone wanting to proceed in that direction.