Is my plan to move and upgrade service panels reasonable?
400A boxes
400A panels are expensive because nobody does that.  The normal thing to do with a 400A meter is immediately split it to two 200A main panels, just ordinary common-as-dirt main panels, nothing fancy at all.  In fact, ordinary/common &quot;class 320&quot; (read: 400A) meter pans provide for two 4/0-sized lugs per phase. That's not for paralleling: That's for splitting to two different main panels. You couldn't attach a single 400A panel if you wanted to, because the lugs won't take a 500 kcmil conductor, and they're not for paralleling.
For some reason, some people with a certain level of electrical knowledge get fixated on doing it some other way (sometimes they cannot believe you can just split to two main panels like that)... and they always wind up with an expensive adventure.
Now, given your requirements, what I would do is (gosh) split to two 200A main panels.  One of the panels would have &quot;Thru Lugs&quot;. The other would be the normal shop panel. So one-a these, and one-a these, coming off any common 320-class meter socket.  Together under $300. You can spend a little more and get the better QO, Siemens, CH etc.  I'm presuming outdoor for NEC 2020.
Anyway, feed the house off the panel with the thru-lugs. It's beholden to the 200A main breaker in that panel, so the run to the house is protected. It has 8 spaces for if you want to tap anything locally off that side of the service.  The 30-space is for the workshop.  I know it's smallish, but feel free to get a 40.
that was easy :)
The neutral bars (plural)
On your original panel, both sides are intended as neutral bars.  They have several features that ground bars don't have.  The panel provides enough neutral lugs for every space in the panel to be a &quot;double-stuff&quot; breaker feeding 2 independent circuits.  You're nowhere near even half that, so you have the option to spin off one neutral bar to make it a ground bar.
However, that's not your only option.  The common way to do that is to buy accessory ground bars for the panel ($8 or so). These bolt to the metal frame of the panel.  You may find that more convenient if some neutrals or grounds are too short.   You can get those (I would get 2), move the grounds to it at your leisure, and then simply remove the N-G boding screw or strap on Upgrade Day.  I don't see one, which is a bit odd.
Routing the cables
Your proposal for cable routing (coming in the bottom when it came in the top/back before) seems perfectly fine to me.  It's clear from your photo that the panel builder left you plenty of wire bending space for that purpose.
Make sure to put a knockout cover on any abandoned holes.