I've not read all your post - I got angry (at the sales man not you) about half way through and stopped.
I'll not go so far as to say he is lying - but he is not giving you good information, and is trying to make you spend FAR more than you need to.
- "don't listen to people on the internet - listen to me"
- because that way I have a better chance of upselling.
- "PC is not a good source"
- Bullshit, though it can be a good idea to electrically decouple it.
- "Get something high priced now because it will be good for the future"
- Probably not because like all tech it will become gradually obsolete. Plus pretty much any well measuring kit you get now will be equally future proofed.
- But what reason is there not to trust a guy who has sold and listened to hifi for 25 years of his life?
- Because a) he is invested in audiophile myths and b) he needs to make money off you.
If you get a decent balanced DAC (eg topping E50) driven direct from the PC and connect balanced to your speakers, that will perform and sound as good as any of the high end stuff he is pushing on you. If you want to eliminate the electrical connection to the PC, a USB to Toslink converter will provide you full galvanic isolation, and totally eliminate any ground loops if you still have any after switching to balanced connections to your speakers (you probably won't). (Your Fiio optical out could do that for you if needed)
If you want to add a streamer to the mix a $100 raspberry pi with one of the standard streaming distributions will give you as good fidelity as any high priced streamer, by connecting to the same (eg E50) DAC. It can also operate as a roon endpoint if that is what you want.
Personally I would not go back to that salesman. He is not interested in getting the best for your needs, he is interested in milking you for what he can.