A comparably-modular mobile phone cannot be anything but thick and heavy, unless you would prefer fragile and unusable.
Android phones have NONE of this. Google could force this. Their "Open Handset Alliance" is not at all open. It's a closed lock in system to keep all their manufactures from ever making Amazon phones.
Google has a vested interested in selling more phones. They could easily require standardization. They could ban binary blobs. They could require all manufactures to make their kernel patches and drivers upstreamable and meet Linux kernel standards. But they don't.
You can take a 32-bit x86 Linux distro from today and run it on an old Pentium. You can take a really old 32-bit distro from years ago and run it on a modern machine (of course very little of the hardware will work, but it should boot in legacy mode...okay, okay it might not boot, but if it does boot you will get a console I/O).
It has nothing to do with the phone and has to do with the fact that ARM is not an architecture. It's a SoC sold to tons of manufactures who all connect whatever they want to onto whatever pins they want to. Yes yes, there are things like Device Tree configs, but name a phone that supports them (no seriously, I'm asking. Are there any? Like I would want to buy/experiment with them if they exist).
Every ARM device seems to have a totally different base image and this isn't maintainable. We need a standard device/IO configuration system so that you can install standard AOSP + drivers onto any phone just like we can with Windows:
If that were impossible, Ara would have not come as far as it did, and the Fairphone 2 would not be as modular as it is.
Soldering can get things pretty small, and pretty close together. Home soldering is possible, but requires some tools, and a steady hand. if you want people to solder, you'll probably need to make the connections a little bigger and further apart than what you would need for a nice smt with mask in a shop. Soldering is is also kind of hard to reverse.
Any mechanical fastener is easy to undo, but much more bulky than soldering. i'm thinking little plastic clips, but there are alternatives. in any case, they're going to be bigger.
That leaves maybe some kind of conductive epoxy and a solvent to remove it. that should be pretty comparable to soldering, but seems like it would have the same problems, you'd want bigger surface mount pads.
mechanical fasteners are the way to go, you don't need any (or many) expensive tools to get it right. but mechanical fasteners are big, and that's space that could go to more battery, bigger screen or whatever. further apart means slower bus speed, and probably other problems.
The other options require special tools. So phone plus $200 or whatever for the tools just isn't worth it. (and making the process super reliable for novices isn't easy)