Currently I have used 2 Fibre ports on ms225-1 connected to ms225-5, should I have gone 1-5 and 4-7 ?
and should I go 1-8 and 4-9? or is that a bad idea.? It is a good idea , but it won't change the fact that If you loose MS225-1 , you lose all connectivity to the MX. It won't add resiliency unless you had an other link from the MX to the main stack. I would suggest MS225-1 and MS225-2 for the agregate and put another link to the MX. STP will block the port but that's fine.
I would definitely consider what @RaphaelL suggested to provide redundancy to the MX - use a second LAN port on the MX to connect to a different switch in the stack, and allow STP to put it in blocking state. I would also split all your aggregations across more than one stack, yes you can do that on Meraki switch stacks. With regards to the new stack, if you have fibre from the new cabinet back to the main cabinet then I would go directly back to it, so creating a star rather than a daisy-chain - means if the stack in the office cabinet fails, then the new stack won’t go offline. And again, if it was me I’d split all the aggregates across multiple switches in the stack.