HIMARS generally makes more sense with something like ATACMS, though stocks of those apparently are precious, and Army is still a few years away from the follow-on.
Having spent time around LCS FREEDOM variants, full-up HIMARS is do-able but reloads are a pain. It got a lot of attention and some FOGOs and activities noticed as 'doing something', but baseline MLRS rounds don't make the juice worth the squeeze.
A deep strike variant might be worth it, but putting a truck that clobbers a big swath of running track / parade ground, er, flight deck (along with its reloads - there was fun around handling those and it's a good thing we can waiver pretty much anything) makes it pretty much a non-starter for more than a photo-op on LCS, and as posted above VLS on LPDs is a better value.
A VL version of that follow-on deep strike round would be a worthwhile consideration. LockMart also had for consideration a 'horizontal launch system' notion for LCS that would have a smaller footprint than HIMARS, but AFAIK it never got beyond a notional stage. We pitched that "HLS" a couple times but mostly for getting big salvos of ASCMs from non-VLS hulls, and aside from handling/storage concerns we took up a lot of the aviation facilities.
As for parking mobile SAMs, unless they get CEC or a waiver for radiating, or don't need more than a data link cue from the ship to engage, that's a non-starter for systems requiring active emitters. There was some talk of 'containerizing' SM-x years ago but that's a long & heavy missile with a round that was designed for VLS (for which it's already containerized), and even if employed unless/until we put suitable combat systems on amphibs you'd need escorts to participate in CEC. So put the demand signal out there for VLS & AEGIS/EASR/similar on amphibs - we have a shipyard looking for ways to sell those self-escorting LPDs.