I will admit that I've also felt like I was losing focus this year for trying to do too many things at the same time, and have not been as efficient as I would have liked. As you said, working on base infrastructure makes you want to write software that's as "perfect" as possible, just being OK doesn't really cut it in the long run, and you want to find time to focus more on the important parts.
I like the separation in 3 components: api, wallet, explorer, it makes a lot of sense. My opinion on each part is:
for the api/platform, you need something that is well-designed and flexible, and eventually very efficient. It doesn't need to be optimized at the beginning, because if the design is well thought out, you will be able to properly optimize later for sure. But you should make sure that the wallet and block explorer can easily be built on top of it. Eat your own dog food :)
most important criteria for the wallet is to be secure. A nice UX is of course always welcome, but again, this can be fixed later. However, if you want devs or advanced users to start using the wallet you'll need to be 100% secure.
block explorer: could be very helpful for debugging during development of wallet, but given that we already have other block explorers available, it doesn't sound as urgent as the api and the wallet.
But I'm sure you already know that :)
All in all, it looks like you've got yourself a pretty solid plan... Good luck for 2018! I will definitely be following closely what comes out of all of this ;)