As someone mentioned earlier, when programming we use mostly opening braces. Double tapping is quite fast, tried rolling before but it was more confusing. It was easier for me to remember "this key is for accessing another layers".

Comma and period are super accessible, quotation marks too. If u are not a programmer maybe other symbols are not so important, but I use a lot of different symbols.

Important part of the symbol layout is grouping

*/+- are for math

" ' ` are different quotations

.,?! is for sentences

:;| is vertical line-ish symbols

It was really easy to remember. for other symbols i have cheatsheet.

Also dead key instead of altgr is much more comfortable.

I have some problems with remembering dead key for capital letters so I cannot say much about it. Maybe I could use it for easier access to second layer.

Also I was thinking about switching my japanese layout to colemak but since i miss my comma and period and have them on dead layer I cannot input it...
Also some letters like "z" are difficult to reach and are used in japanese sometimes.