>6 MS 210 access switches (stacked)
You can stack MS225 and MS210 switches together. MS225 switches have 10Gbe uplinks. So if you varied your parts list slightly to:
4 x MS210
2 x MS225 (all 6 switches stacked together)
You could run dual redundant 10Gbe links back to your MS425. If you don't care about having the uplinks spread across two switches you would only need a single MS225.
I would also consider creating two stacks of three switches, which each stack containing:
1 x MS225
2 x MS210
This has the same number of switches as before. When you upgrade a switch in a stack it upgrades all stack members and reboots them simultaneously. If you create two seperate stacks like this if halves the blast foot print. Also it a stack should malfunction for some reason it again only affects half of the ports.
If it was me, I would create two smaller stacks for the access layer rather than one large stack.
Also here is the campus switching design guide:
https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/...