We all have our own approaches, but it seems like you're getting overly hung up on labels. I mean, if you're under-funding your retirement accounts because you're concerned about having the flexibility to deploy capital opportunistically, the ability to borrow against the retirement account (assuming it's an account where this is an option) largely undercuts that rationale. You have X dollars in earned income that you can either pay taxes on now and then deploy as you see fit, or that you can direct into a tax-deferred account and (again, assuming your plan allows this) borrow against if you want to invest it in something that can't be held in the retirement account. I'm not suggesting there's a "right" approach, but whatever decisions you're making are better made with an accurate understanding of which trade-offs are real and which are illusory.
Borrowing from your 401k is a bitch. I believe the IRS also limits it to 50k max. Wasn't so much of an issue for older people, but that is pretty limiting now given housing prices. Honestly what is 50k going to do for me when an apartment I actually want to live in NYC is $1.5M and requires a 300k down payment at minimum.
A 401k isn't a very efficient investment vehicle. Better to be creative in your tax planning after getting your maximum match.
That said, I sit near the top of an bottom of the next income bracket because I switched jobs this year. I try and push the money that will be in the higher bracket into my 401k to avoid a ~30% tax hit I will take on it.