I've installed Proxmox VE 5.2, but as far as I'm aware it uses thin provisioning out of the box. Is it possible to use fat/thick provisioning? I basically want to allocate all the disk space in advance for each virtual hard disk.
yes. you can replace the LVM thinpool with a plain VG, or with a thick LV that you mount somewhere and use as directory storage. note that plain LVM does not support snapshots in PVE, a directory storage does if you use qcow2 for your disk images: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_storage_types
it's one way to do it yes. I would add 'is_mountpoint: 1' to the definition of the 'local' storage to prevent PVE from activating the storage (and creating the default sub directories) unless the LV is properly mounted.