Please suggest an actual path forward, an actual plan that is more than just "figure it out". And the plan needs to address at least 1/2 of the points I made above.
It's a "Hard Problem". The answer needs lots of time, likely money and at least two humans with strong drive to fix the problem.
It's almost like it's not a technical challenge, it's that getting good looking icons would require a unified userbase, and Apple has that but Linux does not.
Finding money and designers is not really a hard problem.
Granted, you can do a 256x256 and scale it down to 128x128, for example, but if you care for quality some details will be lost anyway. So that's why nowadays you'll see most icon themes are just a bunch of logos plastered over a shaped background.
And what irked me the most was that a few weeks after that I released that first set via deviantart and opendesktop.org there were websites that included them in their sets and made them available for download in their websites, not even a redirect to my deviantart or opendesktop pages or something. And found out after that that some people were using them in commercial projects and stuff so I had to chase them asking to not use them since they were cc-by-nc'ed.
Never got a single cent of any of that. I love making icons, at some point I was even working for the icons that would eventually become the Breeze set for KDE5 with their VDG, but it happens that I also need money to buy the beans.
I would love to encourage you to free your own icons from the round-rect jail. You have some fantastic designs there.
To the blog's point - many KDE Icon Packs have non-uniform shapes (ex: I'm currently using Newaita)
Making good products means lots and lots of drudgery, just for fun volunteers aren't going to touch that, and the stereotypical FOSS contributor is the type that's clueless about UX and puts stability above everything else.
Have fun convincing someone feature x is too overengineered to be usable by anyone who's not an alpha geek and should be simplified to a single switch. Not to mention proper large scale usability testing likely being unaffordable.
So designers stayed far, far away.
(They still have different shapes, though)
I mean this throughout the whole process. The only standard illustration file format I can think of is SVG, but it's largely a format to export to, not one industry-standard software uses as it's main persistence format.
So for starters, contributors tend to need access to speciality software they probably don't have installed to view and edit the source of truth. This also means you're handling at least two files in your VCS, the closed format acting effectively as a blob, no diffs, etc. and an export file (usually more, for different scales) to actually interface with the rest of the ecosystem; this is the file everyone can open, inspect and compare, the one your build consumes, etc.
This already would be a good amount of friction for someone familiar with the tools, but designers are not necessarily familiar with git, the PR process, etc. Add to it that icons are more subjective than code, which overall should follow certain rules and either works or doesn't, and it overall seems not worth it for a casual contributor.