Intelligence augmentation is generally regarded as relatively safe (and thus good to come before AI) but relatively difficult (and thus unlikely to come before AI). See Nick Bostrom's "Paths to superintelligence" in Superintelligence (2014).

The clearest strategy that comes to mind is to make artificial neurons communicable with biological ones and then integrable with whole human neural networks.

This doesn't make much sense to me; I'm not aware of relevant work or reasons to believe this is promising. (Disclaimer: I'm not familiar with intelligence augmentation.)

There are discussions of improving intelligence through genetic enhancement technology. Superhuman intelligence inside humans would have a better shot of being aligned with human values. I'm not sure about BCI though. Here are some examples of discussions of genetic enhancement of cognitive ability if you want to research further:

[1] As the other commenter Zach Stein-Perlman noted, there is a section in Nick Bostrom's Superintelligence books where he describes enhancing human cognition.

[2] Nick Bostrom and Carl Shulman have an article entitled "Embryo Selection for Cognitive Enhancement: Curiosity of Game-changer?"

[3] Steve Hsu, physicist and co-founder of Genomic Prediction, discusses cognitive enhancement. You can see his article "Super-Intelligent Humans Are Coming."

[4] Polymath Gwern Branwen has a very comprehensive article about genetic enhancement entitled "Embryo Selection for Intelligence." He evaluates the costs and benefits to different kinds of enhancement technology. 

[5] A group of 13 researchers published "Screening Human Embryos for Polygenic Traits Has Limited Utility" in 2019. It discusses some of the current limitations, namely the limited ability to predict IQ. More research is needed to improve selection for intelligence. Height is further along.

[6] The other limitation is the number of embryos. The number of embryos that can be used will increase greatly if it is possible to have in vitro gametogenesis in humans. This has already been achieved in mice. You can see Metaculus estimates for this. 

The extent to which you are excited about this might depend to some extent on what you believe will happen with artificial intelligence and how quickly. If we are all gone in 10 years, it might not matter much. If we have hundreds of millions of aligned superhuman brain emulations doing research, then maybe it doesn't matter as much how quickly we achieve these. However, if you have longer AI timelines, then it might be very useful in creating aligned artificial intelligence or mitigating other disasters if we have many superhuman geniuses, especially if they are morally enhanced. 

I have an article that discusses how the political and social environment might change depending on how possible genetic enhancement scenarios might play out. I have a few other articles defending the practice. I'm really interested in this stuff, so you can message me if you want to discuss more.

On another note, I disagree that there aren't projects specifically targeted to enhance human intelligence. I wouldn't discount more mainstream and broader-target projects, including subsets of cognitive psychology (esp. heuristics & biases), developmental psychology, metascience, social epistemology, economics, educational technology, and more.^[1]

More promisingly, I think projects like prediction markets, QURI, the Sequences and LessWrong more generally, note-taking/knowledge network software, spaced repetition software, are extremely promising and I predict they'll have immense impact even if society continues on a gradual path indefinitely.

1. ^{^}

   During the development of the Internet, there were many projects designed to improve collective intelligence, and I predict a lot of that is in the water already. We just can't see most of it because we take it for granted. Wikipedia is perhaps the most obvious example.

(Update: I think I disagree with what I'm saying here, but I think it's worth saying.)

There are several things to say here, but I think the most important one is: superintelligent humans are not aligned.

If you take a random human, or even my neighbour, and you magically give them the power to do any optimisation task faster than any collection of humans and machines presently available, I would be very scared for the obvious reason that most humans kinda suck. This is sufficient as a counterargument imo.

But the more fundamental problem is that, depending on how this magical intelligence boost took place, I wouldn't even trust a superintelligent version of myself. Having that high of an intelligence changes my umwelt and the set of abstractions I can assign utility over.

Presently, I care that others are happy and that they have their wishes fulfilled. For myself, I care about what kind of "story" my life ends up being. I want to be a good book, as judged by my own quaint sensibilities. Perhaps most philosophically annoying is the idea that I want to be able to determine my own story as myself, via the exertion of "my own power". 

But what happens when I discover that my notion of "a wish" is so confused relative to underlying physical reality that, given a much more precise grasp of reality, I have to make some arbitrary decisions about what my original notion supposedly refers to? How would I rescue my values from one umwelt to another?

"Making someone superintelligent" isn't as straightforward as locating a variable in their program, multiplying it by 1000, and leaving everything unchanged. There are degrees of freedom in how you'd implement the transformation. And for most persons, I'm not sure there even are ways of doing it without what basically amounts to killing the underlying person.