It's a lot, for sure. :)

I don't like the letter layout intuitively. I don't type German myself, and it might be good for that. Even so, most German typers will use English too these days and you don't want to learn two layouts. I use Colemak for Norwegian even though it's not optimized for the language; there's that old topic showing that Colemak is actually quite good for most major latin-script languages at least!

For Greek letters (beyond α-δ) I use a switch to a phonetic Greek layout. If you type single letters a lot you can set a temp layout switch key in Linux at least; if you type words you can make a toggle switch. That was is better I feel, and also lets you get other scripts similarly.

A NumPad layer is a good idea, as is easy bracket access if you code a lot. I have a NumPad in my Linux mappings (using Caps+AltGr) but not the PKL ones because PKL doesn't support more than one Extend layer out of the box. Even so, I haven't used it much in Linux either. Things can actually get too complex and chording too many modifiers gets less than charming at a point. ;)

I usually backspace with the Extend one! I press Caps with the left pinky then O with the right pinky.

This means that the total stretch to backspace is the same as when using Caps alone; the only additional hassle is the chording with my right pinky which is really simple I think and doesn't really feel like extra effort. Another benefit is that the Del key to delete forward is directly above (Caps+;), and I can also delete words (Ctrl+BS) with Caps+T+O.

To each their own, of course: I won't touch the <LSGT> key as I need it for my Ø. This is why I advocate modularity, so that users can choose each component of their typing experience separately from the others. In that context, using a separate Greek (phonetic) layout with a temporary or toggling modifier of your choice is just as powerful as and more flexible than using a Greek layer with a predefined modifier in your main layout. If I needed Hebrew or Cyrillic script instead of the Greek one for that modifier, it'd be only a layout choice away instead of a whole remapping. And if I wanted to use another modifier, likewise.

I haven't used the Dash a lot, nice as it is, since my home computer is so slow it grinds to a screeching halt every time I hit the Super key.

But yes, I can confirm your problem. And no, I don't have an answer to it. Ah well, you'll have to use the real arrow keys for the Dash then it seems.

We're really stretching the limits of what XKB was made to do with the Big Bag. I haven't found a way to generate on-the-fly modifiers that work in all applications (the AST keys should've been Alt/Shift/Ctrl with Caps, you know). In some cases some of them work (the Alt seems fine mostly), but I really miss pressing Caps+S+T+N to select the previous word for instance.