Espruino Pico 'adaptor pack'?

What's the third up from the bottom on the left?

Yep, MicroSD. It's the one the original Espruino uses, from Seeed's OPL - but yes, I'm going to have to link to suppliers of all those parts.

Good point about the decoupling cap. I've done a board previously and it works fine, but it wouldn't hurt having the pads there :)

Also, I don't like the look of those really narrow strips between the boards

It's just that each board is its own eagle design, so it's a pain to get the sizes exactly right. We'll see how it turns out!

perfboard is fine with a little wire wrap experience.

:) It's not perfect, but it's better than nothing. On such a small area it also gives you a choice of which orientation you put the chips in - but yes, if you have a massive protoboard then having some proper connected pads is better.