Do you want to upgrade to more I/O or in overall quality?
* For more I/O an X32 is the obvious way to go, you know the system and it seems to work well for you.
* If you are going for other mic pre's you might want to consider starting getting a few bespoke mic pre's and using the line input on the XR18 until you got enough of them, and then maybe switch to another converter.
* If you are looking for other converters right away it's going to be an expensive journey getting that much I/O and mic pre's while upping the quality in a noticeable way. If you are going down this route I would recommend 2-3 Apollo X8p or similar offerings from Apogee & RME. It will cost a small fortune.
If I were you, I would perhaps upgrade to the X32 and then start getting a few nice hardware mic-pre's to get a bit of differing "colors" to chose from. When I got enough of those and maybe a used analog mixer for monitoring and I don't actually need the X32 anymore I would get a bespoke interface, something like the Apogee Symphony or UAD Apollo X16. The converters in the X32 are alright, their weak point is the mic-pre's so that is what I would look to remedy first.