Substrate have a life expectancy before being depleted of its nutrients and buffering capabilities at which point it will need to be replaced. People may have varying opinions on the matter but the fact is, its your livestock and how well they thrive or not is purely up to you.
The substrate chart referenced will provide you a basis as you will see they range from 12 - 24 months.
Shrimp keeping is different from other types of aquaria. A canister filter may do wonders to clean tank water for extended periods of time while harbouring a large bed of beneficial bacteria. However, shrimp are constantly feeding and have no access to the mulm and bacteria in the canister which is why Jimmy kindly suggested adding a sponge filter. Sponge filters serve to add surface area for mulm and beneficial bacteria to grow while providing a dietary supplement to graze on.
You can continue to siphon and perform a deep substrate cleaning however the benefits of the soil are likely depleted at this point so it is basically as good as regular tank gravel at this point with a healthy bed of bacteria.
If you want your Crystal red bees to do well, try to get your water PH down to the 6.2 - 6.6 range while keeping kh/gh within range. PH buffering by the substrate isnt an option with your current setup so you can pre-filter the water running it through peat to lower the PH.
Consider trying the peat filtering first and a substrate swap out as a plan b if all else fails. I know its a tremendous amount of work but figure this. your tank was a high tech planted tank which has since been repurposed so the accelerated growth of yesteryear likely depleted any buffering capability of the soil. You already got a good deal of worth out of the soil. What you can do is swap half the soil out first, then wait 2 months, then swap the remainder so you can keep the tank running.
The options are there but they all require a bit of work and patience. The fact that you are asking for advice shows that you want the best results.
Jimmy mentioned rocks potentially affecting your tank. i read the sieyu rocks raising ph and likely other types may as well. just be aware.