Something I’m looking at myself is HP elitedesk 800 gen3 Tower. Mobo has 5 sata ports in addition to 1 nvme port. It takes 6/7th gen cpu so you could reuse your current cpu. 4 ram slots DDR4@2400.
What is especially appealing is the 5.25 slot for which icydock has a powered bracket that takes and powers 4 SSD disks. (Edit: ejectable too). Sure the chassis has internal space to accomodate even more HDD.
With Proxmox on the nvme + some raidz zfs action would make for a nice HCI setup. My next build and might inspire yours.