For your new tank, do not add KH and do not use tap water! You are just shortening the life of the substrate! Active soil (e.g. Amazonia) will remove KH from the water and will drop it to zero (or thereabouts). Adding KH is fruitless as the soil will just keep removing it, at least it is depleted and then you have lost all the buffering ability.
If your 3 week old tank has Amazonia substrate, then the pH should be below 6.0 at this stage, settling to around 6.4-6.6 'ish after a few months. If you're adding KH or using tap water then you are raising the pH only for the soil to absorb the KH again and bring the pH back down. Most important thing for shrimp is stability of pH. Chances are you have already used up some of the soil's buffering ability, so my advice would be to stop adding KH, use RO water with just GH+ added, and just let the tank settle and stabilise. See where the pH settles; if you're lucky the pH will come down to low to mid 6's. If the buffering capacity of the soil has been partly depleted, then maybe mid to high 6's to low 7's (but I wouldn't have thought you could have depleted it that much in only 3 weeks).
For your older tank, at 2 years old and having used tap water / added KH, think you can assume that the buffering capacity has been completed depleted. Again, just use GH+ in your RO water (don't add any KH) and see where the pH settles over the course of a couple of weeks after a series of smaller water changes. This tank probably won't get down to 0KH if the soil is exhausted.
Don't try to fight the active substrate! Let the KH be zero and see where the pH settles. Stability is the most important thing. Shrimp do not generally need KH with an active substrate.
A pH of 8 still seems a bit high to me for a KH or 3-4.....?
I've set up 5 shrimp tanks over the course of the last 6 months with Amazonia substrate and have done a lot of learning about soil / KH / pH / shrimp in that time, and have made lots of mistakes! For reference here are where my tanks sit now in terms of KH and pH, just to give you something to compare your tanks too and give some range to pH values to be expected:
*Planted fish tank, 6 months old, started using 4GH and 2KH tap water plus added crushed coral to filter to try and keep KH up (doh!). Buffering capacity of soil now fully depleted, KH sits at 3-4 despite not having added KH for months (assume somehow leaching back out of soil or maybe the rocks). pH sits at 6..8 to 7.2. This sounds very much like your 2 year old tank?
*Shrimp tank with moss, 4 months old, RO water and GH+ from start, added little KH+ at start when cycling to try and keep pH up as struggled to get it to cycle at low (<6.0) pH. Now sits stably at pH 6.6, 6GH and 0KH. Think this is fairly representative of a mature Amazonia soil tank in which buffering capacity has not been depleted.
*2 month old shrimp tank, RO water with GH+ only from start, never any KH added, was just patient this time and let the cycle slowly (1.5 months) happen at the low pH. Now stable at 6GH and 0KH with a pH of 6.0 or maybe less (my API test kit only goes down to 6.0, but I'm guessing actual pH is about 5.6-5.8 from what I've read about the soil). This should be fairly representative of a new Amazonia soil tank where the substrate has not been depleted at all.
Hope that gives you some context?