Occasionally, we get a "could you please do a post about..." that's actually a topic that's near the top of the hopper. Today was one of those times.
Today's question was about thru axles versus quick releases as a general topic, and switching from rim brakes to disc. Let me do this in bullet point format.
1. Rim brake hubs are rim brake hubs and disc brake hubs are disc brake hubs. You could use a disc brake hub set on a rim brake bike so long as the dropout spacing matches. On the front, you have a pretty good chance with this. On the rear, not so much. Also, your dreams of 20 spoke front wheels are dashed (DASHED!!) since 24 is the minimum anyone you want to talk to is going to put in a disc build.
2. "Thru-axle" is no longer, if it ever was, a catch-all description. You need to know the axle diameter (generally between 9 and 20mm), and the axle length (100 or now 110 for front; 130, 135, or 142 for rear). There are also different attachment mechanisms, but so far as I know, they are all bike-specific. By that I mean that any 15x100mm hub will fit on any bike that takes a 15x100mm hub.
3. Rotor attachment methods are a separate deal, the rotors you get will have to match to the hub's attachment format, which is either 6 bolt or center lock. You can adapt a center bolt hub to use a 6 bolt rotors are reasonably cheap and they are wear items. We WAY prefer center lock for a number of reasons, but that's a separate story. Your hubs do not care how big your rotors are, but your bike does.
4. Thru-axles mean that there is no inherent connection between the hub and the bike - there is no angle at which the hub will ever nest into the bike's dropouts. The bike HAS no dropouts - it has holes. There IS radial/shear force on the axle of a thru-axle setup. The axle, which is also the attachment mechanism, bears the load. This is what a thru-axle hub looks like (in a laced but not-yet-tensioned wheel).
5. By contrast, a quick release hub (pictured below) does have a mechanical connection to the bike. It is an incomplete one, as without a skewer the wheel will fall out if you lift the bike up, but it is one nonetheless. The hub's end caps capture all of the radial load, and the skewers, which are the clamping mechanism, take none of the radial load. They are purely a clamp.
6. The possibility or ease of converting from a quick-release format to a thru-axle format, or vice-versa, is totally dependent on the hub. On some, you just switch endcaps. On some, you switch out bearings and maybe axles. Conversion ease is not necessarily a universal benefit. It can often mean that compromises were made to the hub's performance purely to make it easy to convert. The hubs shown here can all be converted back and forth - much more involved than just trading endcaps, but you don't need to be a high paid bike mechanic to be able to do it. It's straightforward.
7. The benefits of thru-axles several. It is a more secure and precise interface between bike and hub, which means if nothing else that your rotors are going to be positioned more exactly every time you use the bike. Especially on suspension forks, it can make the steering much less sloppy. There is no stress riser created where the axle transitions to end caps. All in all, thru-axles are pretty nice. In my rapidly-becoming-extensive experience with road disc, I have QR and thru-axle would be fine and dandy but unnecessary. On the other had, they would be nice on my cx bike and I really kind of wish my mountain bike had them.
8. MOST IMPORTANTLY, thru-axles mean I have to use one of these in order to build your wheels. Fortunately, Abbey Tools makes such a nice tool for the purpose.
This is far from comprehensive but it's a good primer. We're way over the word limit.