The OP was about wanting to use internal storage. That’s normally a good indication people don’t want things hanging out of USB ports.
NUC’s are historically great; right up to the point LSPCON chips appeared and started causing HDMI problems. They remain great; as long as they work. However we see continued issues that go unresolved in drivers and firmware, and if we can’t guarantee something will be hassle free we can’t recommend them.
My normal go-to recommendation is an RPi5; never the highest spec, but with the best software support. That’s not really fitting the bill though, although these days there are some crazy ‘PC’ cases that you can run a pile of storage from.
I come back to “if it works, don’t fix it” so keep the current HTPC and just get a NAS to handle data. I’d always recommend using a proper NAS not DIY kits or a pile of USB drives, because I value my data, and I want to use a NAS not add more to the list of things I have to actively maintain.