I believe you're approaching this from the wrong direction.
You have a (currently) useless device for which you're looking for a use case. This usually results in some toying around (which might be fun) but finally turns into frustration because there was a reason why the device was sitting in your drawer.

Try it from the other end! Is there some IT project you'd like to do?
If so, can it be done with an N900 in a reasonable manner? If yes, great! Do it!
If not, put the N900 back in the drawer and get whatever hardware is required to fulfill the project!


As for your ideas:
1. What kind of server? Why would you want to set that up on a decade-old phone with virtually no RAM, a CPU that runs at a snail's pace and no suitable mass storage?

2. The "laptop power" of the N900 lies in it being pocketable while still providing an actual keyboard and a highly precise touchscreen that even allows to control desktop applications which often require pixel-precise cursor positioning. You lose all of that when hooking it up to a TV, keyboard and mouse. There's nothing an N900 can do better in that scenario than any 15 years old x86 laptop. Plus, the TV output of the N900 is just garbage by today's standards.

3. I have no idea what an "encrypted wallet" is supposed to be, if not a USB stick with encrypted data. Sure, you could put a fragile 256GB µSD card in it, but still I don't see why that would be better than any good plain USB stick, or even a HDD if we're talking about backups, where reliability is key.


That being said, there was one scenario I could have imagined for the N900 in the future:
My father collects and refurbishes vintage radios. These all only have FM (and AM, LW) receivers. There's been discussions in Germany to shut down FM broadcasts in favor of digital radio. In that case the N900 could have worked as a relay to feed the radios with FM signals.
But lately it has been announced that FM radio will continue for at least another decade, so this plan is off the table again.