900W should be more than enough. A lot of power requirements are overstated at the MAX power draw not the typical.
For instance my 5G gateway has a 45W power wart but, when putting a power meter to it the actual draw is less than 5W. The issue is it needs a 15V signal to trigger power up.
My laptop says 180W but, typical surfing only needs 60-65W, when the GPU is engaged though for video / gaming it does hit that 180W mark.
I like to find ways to power these sorts of things in a portable manner so, investigating the needs is just part of the deal. Sometimes it's just easy and sometimes it requires diving into the specs w/ a meter. The 5G GW was an issue as I had a power bank and some other things to test it with but, until I put the meter inline it wasn't obvious that it needed 15V. The power bank didn't have a 15V power profile and thus failed to bring it online. I got a different PB and it works great and provides over 16H of power.
There are also USBC >> DC cables you can potentially use. I usually see 3.5 drives as 10W/ea but, the CPU in the NAS might be above 100W which would make it hard to do over USBPD.