As you're using a dechlorinator which addresses chloramines as well as chlorine, that's good, as otherwise that may have been a possibility accounting for the results.
Your tank may not be cycled yet, especially if you aren't getting any level of nitrates. What is the nitrates reading of your tap water? What is your postcode's water hardness level in CaCO3 or German degrees hardness, according to your water/utility website? [Although unlikely, especially with the ocean rock in place, and due to the neutral PH, it's possible that you may have very soft water which can stall a cycle, so worth establishing whether or not this might be the case.]
Is your test kit in date? Does your API test measure ammonia AND ammonium? Ammonium is much less toxic, and I've read elsewhere that a reading of 0.25 on the API kit is not uncommon, while sometimes the colour and therefore the actual reading can be different depending on the light it is read in - maybe worth checking in different lights and getting someone else to see what they read it as?
Scottish water website for some reason advises there is no data for my area! Strange! But I know Scottish water is soft and if I google water hardness for my area, it advises Soft with 25 parts per million calcium carbonate - if that helps
The test kit measure both NH3 and 4 combined so a way to test separate would be beneficial
The test kit expires 2023.
The day after my water change the reading is 0 then on 48 hours then 72 it’s 0.50 I’ve never let it go higher than this and have never let it go past 3 days with ammonia.
I’ve not had any loses and no one looks stressed and they’ve even had quite a few batches of fry.
That's helpful, and the test kit is well in-date so that's not a problem.
The zero nitrates reading might well be accounted for by the similar reading of your tap water, so I wouldn't be overly concerned about that or that it necessarily indicates the tank hasn't cycled, now that we know this.
As for your water hardness, though, that's very helpful - it's slightly softer than my own water, and I had trouble cycling, even with limestone rock in the tank. I think it might be worth increasing the water hardness generally, done very carefully so as not to adversely affect/shock the fish, as this would help the cycling process if it's not complete but also help meet the requirements of your platies who thrive in hard water. This can be done by adding bicarbonate of soda, calcium blocks (e.g. Aqualibra, from Amazon), coral sand in a mesh/muslin bag in the filter or coral substrate; I don't have personal experience of any of these methods so you may need to do a bit of reading up on the various merits and potential downsides of each of these but the key is to ensure that you can control and minimise too much fluctuation.
I would keep going with the regular water changes, and a very gradual increase of your water hardness (you'll need KH and GH liquid-based test kits to monitor this, though) to see if ultimately the result stays at 0 - hopefully this might be achieved within a few weeks of gradually altering the water hardness.
You're absolutely correct about higher PH increasing the toxicity of ammonia. Therefore, a product which can increase the hardness but not affect the PH would be ideal.
The actual ingredients in the Seachem products and the Microbe products seem to be a closely guarded secret. My LFS believes they are the same ingredients although my personal experience is that the Microbe products leave more of an oil slick/film on the water surface than the Seachem ones so I don't think they necessarily are the same. There might be no harm in swapping to Seachem, see if that makes a difference until the tank is definitely cycled (or the ammonia level drops and remains at 0 or even 0.25), then you can always use up the remainder of the Microbe products once passed that stage.
I'll PM you a link to another thread from elsewhere which sounds like a similar situation to yours - worth a read.