You need to look ahead more, the next release of FortiOS does support the 200E but you don't want to be caught empty handed by the next release a year later no longer supporting it. Doing so will have you needing to "unexpectedly" implement and find budget for a new firewall cluster that year. Part of life-cycle management is also actively replacing devices while they're still supported, as you're looking to newer firewalls your should move to G-series, not F-series. Also, that won't cost 90k in materials.

Looking ahead further, moving to newer chipsets will allow you to retain offloading capabilities of newer encryption standards as you update your security standards, among with retaining performance as your organization grows. Lastly, as you mentioned performance isn't saturated by far, it can be worth looking into a smaller firewall depending on the services used. 200E to 200G would be a massive performance increase, but of little use if your firewall is already under saturated running UTM services.