Yes. It maybe there's something in one of the numerous isometric artwork sets available on a download site such as DriveThru RPG that would help, but there are a LOT of those to try to hunt through, and you'd need to convert the artwork to be symbols in CC3+ first, of course.

Top down for the mountains would seem the better fit for what you're trying to emulate. Plus that would save you having to draw more symbols, and just set up a drawing tool with some effects!

One alternative you might try would be not using the RGB Matrix effect, but setting up a simple rectangular polygon that covers all the map on its own, new, sheet, setting the polygon to have a solid red colour, and then adding a Transparency effect to that sheet. If you move the sheet to be below the text, but above the rest of the map sheets, that might work. You'll probably need to keep adjusting the Transparency, and probably also the colour of the polygon (likely somewhere in the reds and oranges range) to get to something you're happy with. Not guaranteed to work ideally, but it should give you more control over the final colour of your text, at least.

Selecting can be problematic, especially when you have two things that directly overlap one another. Fortunately, there are a number of different options you can select by using CC3+ to identify which of the objects you've selected you actually want to do something with. This post, the accompanying short linked video, and the linked additional post, again all by Remy Monsen, will help with this, whatever issues you're having with selection.

There isn't a global search mechanic across all the add-ons you may have installed, mostly because it's advisable to choose a style to map in, and stick with that, since those items will usually work well with one another in terms of the overall map appearance. That doesn't stop you from bringing in symbols or bitmap fills from other mapping styles, but you'll need to identify what those are when doing so first of all. If you want to see what symbols and bitmap fills you have on your system, the easiest way is to use Windows' File Explorer, and navigate to wherever you have your CC3+ system installed (the standard default is in ProgramData under Profantasy, for instance).

Don't forget that aside from adding more character to the coastline, you can also add smaller islands there in places to help break up that "smoothness" aspect, much as other features too small to have been added on the original larger-area map can be added inland. What's shown as a peninsula on a large scale, might be really an island with a tidal causeway headland instead when you start looking closer, say!

I'd definitely recommend using your Atlas mapping to experiment with different styles; it's what I've been trying to do for some years now! It does indeed push you into thinking in different ways about mapping, and trying out new things in terms of how to use the program.