I'm somewhat pragmatic when it comes to 'custom' , since I consider any tube-to-tube construction potentially custom. I.e. if you ask the builder to make something with a specific geometry then that's what you should get. I've done 50 some odd 'custom' frames just in the last few years in Japan. All from Titanium and welded up by an aerospace company in China. (And Taiwan). No fuss, no muss, exactly to my specs.
The best builders are not too up about all this - you order the frame, they build it , and then move on. Unless you are requesting a UN Heritage Art Piece, or the builder thinks they are that class of artisan, the frameset should just reflect the specifications from the communication you have with the builder.
All builders have their specialty and niche - so the best way to select is to determine what you want, then find out who builds it.
If you're purchasing for investment value, then find the builders who are selling for the most and maybe towards the end of their careers.
I don't believe in the 'perfect bike'. As a young racer we had many frames available for any conditions, and we changed them alot. In one season maybe we each had 5-8 bikes. This was pretty normal and easy to do because, yeah, well, the frames were all 'custom' built to our specs by various builders. Japanese Keirin is similar. The builders must build a set number of frames on contract to JK to keep their license. So, they end up building alot of bikes. If you've raced track, you'll know how hard it is on bikes. So, this is why the Keirin builders generally build pretty darn good bikes - they build alot of the exact same kind. Small innovations are snuck into the build, the devil is in the details as the frameset itself is highly regulated.
From the US builders you'll find alot more structural innovation. And trends into the gravel, all road, CX, tour, whatever. With many many variations and some similarities depending on West Coast or East Coast (or Montana).
Why not buy a few different bikes? Then keep the one(s) you really like? Until you actually get a bike built up and ridden a few hundred or thousand km, it's quite hard to say if it really 'sticks' or not. And if it's your first steel type bike, you may wish to start with something a bit more stock until you know what niche you want to settle into. So, for example a stock Nagasawa or Kalavinka - these are well under $3000/frameset and hold their resell value so if you don't like it - it's easy to sell.