Not having a separate mobile theme is good news. The current mobile theme looks good, but lacks functionality compared to the desktop version.
Have you considered taking donations to offload some of the other tedious work (assuming that there's more that could be done by 3rd parties)? I'm sure there are many of us that would be happy to donate to help reduce your time and effort with this upgrade.
Yes, being able to use the same themes on phones, tablets, and desktop computers is ideal. This means that not only can people use the default AtariAge theme, but also the others that will be installed (such as a dark theme). And you don't lose any functionality.
I may completely drop support for Tapatalk, as it sounds like people still have issues with it and seems unnecessary now.
As for taking donations, I'd be happy if people purchased a forum subscription, which helps defray the costs of running the site. I may make the subscription options a bit more obvious than they are now once I get the site moved to the new server and everything settles down.