I got a tiny Lenovo M720Q (i5-8400T / 8RAM / 128NVME / 1Tb 2,5" HDD) that I want to set as my home server with the ability to add 2 more drives (for RAID5 if possible) later using its two USB 3.1 Gen 2 (10gbps).
Do not use USB drives in a multi-device scenario. Best avoid actively using them at all. Use USB drives for at most daily backups.
I wouldn’t advocate for RAID5. I’d also advocate against RAID to begin with in a homelab setting unless you have special uptime requirements (e.g. often away from home for prolonged periods) or an insane amount of drives.
I will mostly use 40/128GB of its capacity with no idea how to make use of the rest.
I use spare SSD space for write-through bcache. You need to make the decision to use it early on because you need to format the HDDs with bcache beneath the FS and post-formatting conversions are hairy at best.
most of what I read online predate kernel 6,2 (which improved BTRFS RAID56 reliability).
Still unstable and only for testing purposes. Assume it will eat your data.