I would consider Fidelity or Interactive Brokers.

You can share a host folder into the VM via VirtIO-FS. https://imanudin.net/2025/04/20/how-to-share-host-folder-with-guest-vm-in-proxmox-using-virtiofs/

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I don’t know that particular card (and can’t see the p/n) but you might be able to use sata drives just fine with a breakout cable (SFF-8087 → 4× SATA)

Yup, easy, just add a config with router, host rule & service specification for each machine with their respective ip as destination.

https://doc.traefik.io/traefik/master/reference/routing-configuration/http/routing/router/