Since Broadcom took over VMware, they’ve been phasing out free ESXi which is a bummer for you guys.
No direct experience but Proxmox VE is free and open-source, with optional paid subscriptions for enterprise support. Migration from ESXi is doable and lots of references online.
HP Microserver Gen11 is compatible with Proxmox and should work well in you case.
I beleive XCP-ng is another good option, alas booting options and configuration I do not know 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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