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.

Welcome aboard Reddog!

Yep, following the tutorials and working through the manual's exercises is definitely the only useful way to start. Critical comments about the program you'll find scattered over the Internet elsewhere are all about people NOT doing this, and blithely assuming the program's going to be identical to the basic graphic manipulation programs they've "always" used before. IT ISN'T!!! This is actual CAD mapping and draughting software, and if you're not familiar with how that operates, you will struggle. I did, certainly, until I watched some of Joe Sweeney's videos (there weren't any others at that stage, as this was some years ago, back when it was still CC3, not the current CC3+).

As far as map scaling's concerned, forget ALL about what size you need this or that bit of the map to be once you have it printed out, as this is IRRELEVANT to the creation of maps in CC3+. What you MUST do is create a map to the actual size the map's supposed to be using the relevant real-world map units - which for floorplan/Dungeon Designer style maps are either in feet or metres (you can decide which during set-up for that map). Make sure everything fits in your map to that scaling only. If the grid squares are meant to represent 5 feet, for example, make them 5 feet square in your CC3+ map.

When you've finished the map and want to print it out, that's the first time you need concern yourself as to what size things will be in the physical, printed-out world. If you need advice on that, or anything else connected with map scaling, see this excellently thorough blog post by resident expert and Forum mod Remy Monsen, which tells you all you'll need to know and more on the matter!