On memorizing: for me, it hasn't been difficult. Specialty layer switches are placed on keys that are difficult to press so I don't accidentally hit them. Additionally, since they're out of the way, it's easy to remember them because of the difficulty.

For more common layer switches, I extensively use dual-use keys. For example, if I tap "F" then I get the letter "f" but if I hold it, it switches to a vim-like layer. HJKL becomes arrow keys, Y becomes Ctrl+C, etc. The point is that you don't have to dedicate more keys to do layer switching. You can have the same key do different things depending on how it's used.

For me, memorizing has been easy due to incremental change. I didn't come up with my new layout on day one then stick to it forever. I'd find a frequent pain point, tweak the layout, and memorize the tweak. It's easy to remember 2 or 3 key changes at a time.

For general typing - I dedicate ~15 minutes a day on sites like monkeytype.com. I also completed the courses on typing.com when I first received my moonlander. Monkeytype also has settings to emulate a lot of the popular alternative layouts if you want to try any out.

For shortcuts - I just conciously accepted it takes time to learn. I'd say it was about a month before I stopped reaching for the old shortcuts and started to use the layered shortcuts.

It can also help force you into it by not having the old keys mapped - obviously this comes with judgement on a per key basis. I'm not asking you to unmap "A" - but unmapping/not-mapping the F-keys in their usual place coerces you into using your new mapping and you'll just get the muscle memory eventually.