[tuvan]

You could just get a custom keyboard that is comfortable for you. Key mapping is generally an independent step. Most of these keyboards are completely programmable with custom keymaps and layers. Many come with blank keycap options, you can also rearrange a regular keycap set.

[enormousness]

You can't do what OP is looking for without a custom PCB. The switches and caps and mappings might be customizable but their physical position and size on the board isn't.

Despite pushing the envelope in many ways, custom keyboards can also be strangely traditionalist at the same time. Making a PCB is a risky venture and most manufacturers don't want to risk it.

A huge amount of models still use a huge 7u or larger space bar for example when something as small as 4u would suffice, and leave room for much more ergonomic placement of modifiers. (or other improvements like extra keys, keyed gaps etc.)

[tuvan]

Yes, most keyboards still have the traditional layout. But I believe many options fit what OP needs. A split keyboard such as Ergodox EZ allows access to the maximum amount of keys possible without moving your hand. You can then customize the mapping to properly distribute commonly used keys.