It is unlikely you can negotiate away the test, because it is something they have decided to expect in the interview process.

Personally, I would agree that their process is flawed - see e.g. A programming task is scaring off candidates, should we ditch it? for much discussion on appropriateness of such a test. Although stannius comments that at least this company has not used a long technical test at an early stage as a cheap filter, so is behaving with more respect of candidate's time than some.

Your counter-arguments are probably not going to hit the mark because of this, and are IMO not worth raising.

I have solid evidence of proficiency in language X by pointing to my side work (in github) over the past 3 years, spanning 100's of hours of my own personal time

How does the interviewer know this without detailed study of your projects, and some kind of evidence that the contributions are actually yours? An in-house test will solve a problem that the interviewer is familiar with and should be able to judge required skills from. In short it is less effort and risk for the company to use a standard test.

Given my desire to convey myself as a rigorous, test-heavy software engineer, it will likely take me ~6-8 hours to complete the exercise. In other words, to put my true face forward, I don't expect to complete the work in ~4 hours unless I give it less than 100%.

I too find that these tests can take longer than suggested, although it varies a lot with different employers. I don't see a valid argument for not attempting the task here though, just a complaint that it is "too much work", and also a potential red flag that you will take double estimated time on real tasks in future because you expect to take your approach to test-driven development ahead of whatever the company thinks is appropriate.

to be honest, I'd rather use my personal time to continue studying my current "curriculum" of side projects, namely more advanced features and OSS in language X

What you want to use your personal time for is not something the interviewing company takes into account, any more than for instance your commute time should you accept the job.

The company may be wrong in their assessment of how useful the interview task is (or they lack appreciation of decent tests and your hobby work). They may not consider how much of a barrier the interview task is to getting good candidates. However, confronting them with that opinion before they've decided to hire you may give the impression you are argumentative, and is likely to reduce your chance of being hired.

Ultimately, you have to play their game by their rules, or decide it is not worth it.

Once you are hired, you may find yourself with enough good will and trust that you can give some feedback on their interview process. Although I suggest waiting until you are either asked or you feel you have built up enough trust that you can speak frankly on the issue. Future hires, and maybe even the company, may thank you if so.