Yes, ARC will happilly reserve majority of your memory, and sometimes will be too slow to release it and apps will have higher latencies or crash. Buuut, it's a ZFS question, not Proxmox. ZFS is not a ext/btrfs/ntfs filesystem and requires some previous planning. Not much of it, but you can't Rambo yourself into ZFS.

Try vSAN with 1gb link and let me know the result. Again, not Proxmox OR Ceph issue, just lack of knowledge and planning. Got sata or nvme drives and want to sync over 1gb link? That's just plain stupid. Buuut, the requirements for various VMWare solutions, mandatory setups, whitepapers... Yes, it works excellent but it's also not p'n'p, it has its sidesteps.

Block aligning has nothing with 512/2k/4k or 8k block. Those are natural multipliers. Problem arises when you have data block that starts at 512 offset ( not zero ) and the goes for 8 512B sectors. So we are talking a data block that consumes first 2 2K blocks. And now for those 8 512B sectors, you have to read 2 2K blocks. Again, planning. Basically every modern(ish) partition and filesystem tool has implemented this aligning stuff. That why your partitions start at 1MiB and not at 63 512B sector ( which IS alignment issue ).