I am confused a little, these dedicated NAS devices are either big, fast and not handy or small and not very useful, in call cases most of them are limited in features, so..
I think each and every one here have such device in his hand, pocket or at hand length, such device capable of everything you want or might need, yes there limitation on how the maximum size of Microsd it does support and most likely it only up to 1TB, but if 1TB is not enough then buy another.
so here my suggestion:
Buy a used and cheap android, one with broken/cracked screen will be perfect as you can get really cheap, put memory card in it and don't use SIM with it, plug it in charger forever, or you can use it on the go when you like and it does have wifi to join your LAN and also it have wifi hotspot if needed, there is hundreds of software and every flavor your can imagine to add and even combine, from
SSH to WebDAV, passing through DNLA... and on top of that you can use multiprotocol at the same time as it is simply sharing the storage, also if it is needed, then install full linux with chroot, or just use termux and install any OSS linux software you like, just in case you didn't like the apps from play market.
After that from the
OS like Windows, just mount the needed space/storage as drive, also there is many software if you don't like the Windows included functionality.