Just for a sense of perspective, my featherweight drisc (180 diameter) has 6mm hardox bulkheads and equivalent of about 16mm disc, and my larger spinner (also featherweight but 300mm diameter) has 20mm HDPE bulkheads and an 8mm thick disc - the latter setup being based on Inertia XL which did a good number of events without breaking too heavily

So in a nutshell I'd agree you're thinking too big haha, 15mm aluminium and a 15mm or so disc should see you sorted I think depending on your disc design. Would also heavily recommend a dead shaft over a live one - it increases the strength of the bulkheads massively for if you take a hit from a horizontal spinner