The price difference between Nano and micro BL is 100% basically if we are talking about new units. xDSD is cheaper. Another advantage with xDSD is that it has great synergy with xCAN if you would need amp later for one or another reason.
The thing is that all of those are running on batteries since were designed for portable usage. If you would like to go in desktop direction and you like iFi I would go with iDAC2. For now this is the best desktop option in micro range (while it is legacy product) according to iFi itself. I have it and I am very happy with it. It also gives you option to extend your setup further with iCAN SE or iTube2 for example (careful thou since in headphone amp setup iTube2 adds very little comparing to other options but you may eventually want it). iDAC2 handles MQA with latest firmware which may be important for you since you stream from Tidal. The thing with iDAC2 is: if you add up all possible pieces of iFi stack: iGalvanic, iUSB, iDAC2, iTube2, iCAN SE and iRack you will have similar price tag to Pro iDSD but without wifi features. On the other hand with iDAC2 or micro BL you can build your systems in steps. I went with iUSB micro for example since I use it with other devices like Mojo for example.
The other thing is that if you like iFi sound I would go and listen to both xDSD and micro BL than I would go and listen to something else just to convince yourself that this is the sound you really love. Hope this helps.
and now it can be not only time consuming but also complicated or even impossible. As far as time goes by: if you plan to spend at least an hour daily per year listening to your setup for next 2-3 years for example, spending even a week on doing right selection makes a lot of sense IMHO. I'd start with budget limit and write down of all functionally needed. Even the measurements of device can be important but be flexible with it as it may turn out later that you don't really need some of functions. I went with iFi micro line since it does not occupy too much of space on my desk and it is modular (iUSB is used with Mojo too at the same time for example) I like the sound signature and SQ at the price tag. When connecting iFi devices together you can feel the synergy between them and it is hard to make wrong choice. Some people say that iTube micro is a disappointment since when adding it as a last step it does not increase SQ a lot. Indeed it adds just a bit, but as price goes up do not plan to have a night and day difference between stages, it growns incrementally and last steps can be very small. So while I agree that iTube adds only a bit (and in case of some settings you can barely hear the difference in headphone setup) and you have to wait a minute before it is ready when I removed it from my setup for a test I wanted it back immediately. 