02/21 Build date here reporting back for folks in a similar vein for just trying to get the button enabled after a DIY install.
I purchased parts OTC in September/October for roughly $255 after tax and such and did the installation myself.
The HEPA assembly is $200 bucks now.
For the software portion this week, I was able to submit a request for mobile service, but the local SC closed it out, mentioning a retrofit was not available. (Old info on the SC side)
I had to ask them to verify if they had checked the latest service bulletins, and I attached a screenshot of a previous person's service statement posted on the forums here that showed Tesla having an official retrofit line-item. This helped the local Service Center realize this is now official, as it seems to be newly rolling out.
Because the first one was closed, I opened a second request, and mobile service came out a few days later to validate the install, and enable the button
(was the next available appointment)
Mobile tech came out this morning, enabled the button, and within 15-20 minutes, he was all said and done.
Most of the issue was needing to do a 12V reset and then the button appeared. - What is weird is that even though my appointment was slated for the button, he also arrived with HEPA retrofit parts to install, even when I had the request mentioned the part was installed. (included a picture of my installation) - may have just been a 8 am coffee brain fluke.
All in all - It was $30.75 bucks to enable the button for a DIY install.
(sorry for all the info, but hopefully helps someone else going through a similar process.)