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Could be worth trying some heat mouldable footbeds - Sole for example https://yoursole.co.uk/collections/mens-footbeds
I agree with Alf0nse that footbeds provide stability in any shoe, but having had custom orthotics made a few times (once for general use, once cycling specific and once after I severed a tendon in my foot!) I tend toward the opinion that the heat mouldable ones give 90% of the benefit at 10% of the price.
I had mine done in Bristol. I got a referral from my previous employer’s physio scheme.
There seem to be plenty offering custom orthotics in Brighton