Couple thoughts
Given that you are building from scratch, I agree vehemently in starting with speakers first. Speakers are frequently the worst component in terms of resale and visually the most prominent combined with being heavy, easily damaged and hard to handle.
You mention "I love planar speakers for their speed and transparency, but the fact that I can't sit still in their minuscule sweet spot for very long, coupled with their general lack of bass probably makes them a less than ideal choice for me."
This is more of a view of one particular brand rather than all Planars. There are others that behave very differently. Take a look at Diptyque. These have MUCH better bass response and overall are an exciting speaker that works quite well and have a broader sweetspot.
For that same sort of immersive experience you could look at speakers beyond Legacy which is a nice product. Brands to consider:
1. Borreson which is a hot at the moment is an interesting product as is. These are tough to place in-room but if they work in your space they are amazing
2. Raidho/Scansonic which is Borreson's former company. Same issue as Borreson, hard to place but have a broader range of pricing.
3. Audiovector as it delivers a similar experience, is easier to place and is far more reasonably priced. That said, they aren't as novel as Borreson at the moment.
There are a million other speakers around that are great. Everything from Martin Logan, Vivid, Wilson Benesch, Wilson, Magico, Rockport, Marten, etc... You need to find ones that you want to look at and deliver the experience you want.
Full disclosure, I am a Diptyque, Audiovector, Raidho and Wilson Benesch dealer. I have no affiliation with any of the other brands mentioned.