I think for me the confounding factor is I'm very unclear on enforcement of any of these. Yes, so the Beis Din can impose collateral until you pay your shekel, but did they? What kind of evidence did they require of nonpayment before imposing collateral? The actual donation process as described in Perek gimel is pretty anonymous, you just put your coin in one of the big boxes in Yerushalayim and then who is going to be able to say whether you actually put in your shekel or if you just put in a piece of wood? Or if you happened to put in your shekel when nobody was watching, how can they even accuse you of not paying? I guess maybe it's easier in the outer parts of Israel where there's a whole process of sending the shekel that gets documented, but still...

I tend to think of taxes as things where the instrumentality of the state is enforcing them, it's not clear that this is the case here, so I'm uncertain about their taxiness.