What does future proof mean to you in this context? Are you using it like a server or just storage? Are you trying to have growth for space, performance, new features/codecs?
Any appliance like that locks you into that ecosystem unless you're willing to move your data. You can buy a bigger Synology and move disks/expand volume etc or you can move your data now and go to something that you can upgrade.
For me the per slot costs on a Synology big enough to handle my needs put me solidly into build my own territory. I settled on ZFS and spent a while on Nexenta Community Edition before eventually exporting my volumes and importing into FreeNAS where I've been ever since.
ZFS IMO is the right place to be if you're serious about keeping your data and capable of supporting yourself. Synology would probably be my choice if I wanted something that just worked, so getting a bigger or more powerful Synology is a perfectly acceptable answer to your dilemma.
Viper GTS