Agree on this....
Have to mention that you picked an interesting sizing. This is fine for a huge chunk of our value range (AK+) and bluffs. But 55 doesn't seem to be too appropriate, because we don't want to force light calls (from hands that beat ours) nor do we really gain calls from weaker hands (for value) or fold out stronger hands.
It looks like a small misconception to me about sizing choice OTT and what we want to achieve.
So I would check 55 here pure and if we think about turning 55 into a bluff (which can be ok with the spade I guess) I guess we have to bet river (big).

Why check the turn?

To me, this is an odd spot because I don't think "GTO" ranges on Wizard, Upswing, Snowie, etc. are going to 3b 55 SB vs MP. So, since 55 is not going to be in any of the ranges, it is a hard spot to model in my opinion.

My logic for leaning towards checking turn is as follows:

1) It is hard to get called by worse when we bet
2) It is hard to get better to fold when we bet
3) We can be drawing razor thin vs made flushes
4) They have a higher concentration of nutted hands on the turn (it sucks to bet/fold here when he has A8ss or sets for example). We redraw vs the majority of those types of holdings on the 2s or 7s.
5) That turn is better for their range.