I don't understand the need for a trapper on this team at all. Yes, all CROMuks enjoy steels being off the field, but it's not a complete necessity if they are dealt with in another way (in this case, a combination of Shiftry and your choice of EQ-carrying mons).
Let's look at the relevant Steel-types who would typically stop Muk's sweep
- Bronzor
- Mawile
- Metang
- Probopass
- ???
Of these, Shiftry squarely pressures and defeats two (Zor + Metang), either Knocking them or defogging on them. In practicality, this may require absorbing a Toxic, but it's well worth it in most scenarios, especially since we are lefties + Synth.
Golem takes care of Metang as well, and destroys Mawile and Probopass, while providing a significant amount of other utility for the team. By choosing it over a trapper, you are only missing out on completely trapping (and, in the case of Probopass, dubiously eliminating) Bronzor, which you already have one countermeasure for (not to mention the fact that we still have 3 mons left after this).
Picking Probopass creates its own issues because, like I mentioned, it's really not all that effective at eliminating Bronzor anyways. Psywave is shockingly always a 3HKO, which means that you have to completely sack your rocker in exchange for theirs after getting up rocks, which effectively removes rocks from the game once one side defogs. In addition to performing poorly as a Bronzor-eliminator, Prob compounds the existing Ground weakness introduced by Muk and also gets completely destroyed by Monferno CC. I understand that Golem can only switch into both of these things a few times, but it's better than none at all.
Alolem, while awesome and fun to use, suffers from some of the same problems as Probopass. It eliminates Zor more quickly, but it always gets toxic'd to do it (which is fine, but not especially appreciated on a slow mon). After this job is done, I have found Alolem to be more of a liability than help in a lot of scenarios; it is too slow to threaten most of the tier effectively and its weaknesses are both exploitable and compounded by our existing composition. Also, going with Alolem limits which rockers we can choose, as we don't want to add another water weakness to the team. Relying on Shiftry as a ground-and-rock switch-in is questionable at best, even on an offensive team.
I disagree that we should go offensive with this team by merit of the core; we have a non-SD/sweeper Shiftry and a lategame setup wincon. Neither of those screams 'offense' or 'defensive synergy be damned' to me.