I think the issue here is that the ScotRail planners are working to a few competing requirements.
There are operational requirements where these units were primarily for the Edinburgh/Queen Street/Stirling area workings, and the Shotts services. There is also the requirement to replace the class 314 units, so they've done so as part of one big order of 385s.
The units on routes working to and from Central are diagrammed partly based on passenger numbers. Hence why the former North Berwick class 380 diagrams were switched to the Neilston line, as the 4 car 380s provided increased capacity during the day on these services.
The class 385s are now taking most Lanarks and are slotting in on the Inverclyde and Cathcart routes. The reasoning here appears to be that as Gourock drivers work the Cathcart routes, the 385s can be worked to Inverclyde with little/no concern with crew knowledge. I think part of the reason they will persist with 385s on the Cathcart routes is that the original plan was for 6 car class 385s during the peaks on the Cathcart Circle, again due to passenger numbers.
Ideally there would be the set up that
@SC318250 has outlined above but there wouldn't be likely to be enough 385s to take over Ayrshire srvices and you wouldn't get any seven cars like now with the 380s. You'd have to train Ayr based drivers on them as well, which until now they haven't been. There's an argument for working them alongside the 380s on that line, releasing some 380s to most likely INverclyde, but it seems they have thought why bother with a cascade like that. I think it will be likely they will do the same with the Cathcart routes, directly replacing the remaining 314s. However, I don't think the 385s are any worse than the 380s for a route of that nature. Both unit types struggle with the short distances between stations.
Again, ideally class 318s and 320s would be better on the Cathcart routes, but there's probably not enough to completely take over, and there would likely be a loss of capacity even if they did. I would also point out, that these units would have to get some of the panels around the doors taken out, as they are not the most spacious for standing.
Operationally this isn't a great scenario as you will now have three unit types on the Cathcart and Inverclyde routes none of which are compatible, which is disastrous in a failure situation as the chances of a failed unit being rescued by the one behind it are minimal. I can only imagine the control are frustrated with this, particularly with the Cathcart Circle where during times of severe disruption, the old Central Signal box could turn trains as required without any thoughts to the consequences and report back later on what had formed what. To do so now would cause havoc with all the different unit types ending up in random places at the end of the day. So for performance it potentially isn't great either.
However, I think ScotRail are largely doing what they are told when it comes to unit diagramming, and the units with the greatest capacity are being allocated to the routes with the greatest demand, with the rest filling in behind almost quite randomly.