Is there any way you can get rid of mpm_prefork? If you're not using PHP, you probably don't need it...
I'd look at scaling horizontally for peak loads: move the database off to its own server so that there's no dynamic data on the web servers, then deploy more when the need arises. Use NodeBalancer or your own front-end load balancing solution of choice to route requests. Also, no downtime required to add additional resources.
This does mean you have an additional database server and a NodeBalancer deployed all the time (increasing MRC), but it does make things quite smooth.
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