It’s extremely simple. Although I prefer ZFS I will give you an example with BTRFS since it’s easier to get going. RAID1 in BTRFS is considered stable (RAID5/6 is not).

sudo mkfs.btrfs -m raid1 -d raid1 /dev/sdx /dev/sdy # Create raid array with BTRFS
sudo mkdir /mnt/storage # Create your mount directory
echo "/dev/sdx /mnt/storage btrfs noatime,compress=zstd 0 0" | sudo tee -a /etc/fstab # Set raid array to mount at boot
sudo mount -t btrfs -o noatime,compress=zstd /dev/sdx /mnt/storage # Manually mount the first time

You would also probably wanna set up a btrfs scrub once per month, either with systemd-timers or cron, whatever you prefer.

It will yeah, although with modern SSDs it really isn’t a big problem. I’ve used an Samsung 840 EVO as L2ARC for 8 years now.

Try changing to another upstream DNS server in Adguard (such as https://dns.cloudflare.com/dns-query) and see if the problem gets resolved?

Always had trouble with Nextcloud but ever since deploying it on NixOS it’s been rock solid. Both performance and reliability.

Personally I would go for ZFS with the SSD as a L2ARC. But among the options you listed I would do BTRFS RAID1 if you’re only gonna use two HDDs, and mdadm RAID5 with BTRFS on top if using three.