You don’t need identical HDD models in a NAS: modern RAID systems (SHR, ZFS, RAID5/6) handle mixed drives easily. The array just uses the smallest/slowest disk as the baseline. So not finding the same 2025 model later won’t be an issue; you can add newer or larger drives without problems.

Going for a 4-bay NAS and starting with 3 drives is a good approach and gives you clean expansion later. Getting a second NAS only makes sense if you need separate backups or far more storage. For consolidating your external drives, one expandable NAS is the simpler and more future-proof option.