Is this a "supplementary relative" ?

First, thanks again for all your help. You guys are the best.

So my gut tells me that this sentence is perfectly sound and grammatical: "To be completely honest, which is a difficult thing for me to be, I really like the Dave Matthews Band." But I'm unsure of the sentence's exact construction.

We have "To be completely honest," which is (I think) a non-finite clause -- an infinitival -- working as an introductory element?

The "which-clause," however, I'm unsure of. Is it a supplemental relative? Thanks in advance!