Help with building a custom split keyboard for the first time

Hi all! I've recently become aware of the rabbit hole that is ergo KBs, and was immediately hooked. After some thinking I've decided that I want to build myself a portable custom KB based on Ben Vallack's custom keyboard guides, following the Ergopad -> Ergogen -> KiCAD workflow. The end goal is creating something like u/danns87's amazing keyboard. As this will be my first custom keyboard, and more broadly my first DIY electronics project, I have a couple of questions - for which any answer would be greatly appreciated.

• Number of keys - as it is my first split keyboard, I didn't want to go overboard with removing keys. So far I thought about designing a 3x6 + 3 layout, probably pretty similar to the Corne, as to my understanding it's the largest layout that preserves the "1 key away from home row" principle. I wanted to ask what was y'all's experience with downsizing, and whether you had at least a decent readjustment period to the new layout. I should mention that I write mostly code and LaTeX (using LyX specifically), and wanted to ask whether that amount of keys will suffice for these sort of tasks, as I do rely on "special" symbols.
• Soldering - this will be my first time soldering, and honestly for me that part of the process is the most deterring. Guides I saw online said that overall the keyboard only requires simple solder work, but many of them also said that in some parts, for example, the iron shouldn't be too hot - without specifying temperature, so I'm mostly afraid of frying or melting connections. Do you know any beginner's soldering guides/tips that are specifically linked to this subject? I am aware of solutions like hot air soldering but I don't really want to buy a specific tool like the hot air gun for a single project without me using it again.
• Casing - I haven't seen many online guides regarding cases for DIY boards. Do y'all find it necessary for longevity/rigidity/portability? I don't have direct access to 3d printers, so is ordering a print of a case a good idea? Would an external casing, like in u/danns87's post, suffice? I will say that I don't have much experience with 3d, and that I know that Ergogen can export an stl for a 3d print, but I don't really know how it works.
  

Over all, are there any new/other methods to improve the design and assembly process? Thank you very much :)