tldr: For Linux adoption it would be better for devs to focus on 2 (“main”) distros which are very similar to Windows and macOS and then 2-3 further (“big”) distros which give a bit more room to experiment. All the other distros create confusion and analysis-paralysis for the user who wants to switch or wants to help others to do the switch.
This is a mythical man-month fallacy. If everyone who works on distributions focuses on just a handful of them, that does not mean development will go any faster or lead to better outcomes.
Also observe that majority of work which ends up in GNU/Linux systems is outside of distributions. And this work often ends up quite focused.
First step is to shrink and possibly move around your existing partitions to make space for a new one. GParted Live may help with that. Once you have enough space, just install Arch there. If you already have separate /home you should be able to reuse it.