This is a little worrying Sherri, because I remember you saying a couple of years ago that to better manage your time and improve things that you would be switching to focus on new themes, a new framework and maintaining only some of the better quality, wider appeal classics, because some of the old ones were getting dated in style and format. You also said you'd not be taking on any custom themes because they are time consuming.
Please, because I've always loved IBTheme, give some thought to some suggestions (not intended as criticisms and I'm not claiming to be an expert):
Perhaps get a few QA testers. Put out at least a release candidate of each theme, in time to get at least 2 weeks testing on a variety of real world devices, if only for those extra special seasonal themes.
I frequently see issues with the Google fonts cropping up, the CSS specifies a certain style, but the font is never loaded, only Lato is, so the end result is a fallback to 1990's style traditional fonts like Verdana and Aria!
You can also load more than one Google font in one request (line of code) which is faster.
DNSPrefetch and Preloading is handy for image or code heavy themes like yours, because they preconnect resources and can load in the background before being called.
Streamline that CSS! It is honestly so bloated, I bet you could probably save yourself quite a lot of time on editing, your themes would be lighter and faster loading as a result.
For example, there are many repeated lines of outdated vendor specific css properties, originally needed for cross browser compatibility that you probably don't need anymore, because they have since become obsolete. Features like gradients and shadows are now well supported across browsers.
There are 100's of entries containing 0px in your CSS code, but there's technically no such thing as zero pixels or 0 em. You could save all those extra characters by carefully doing a search and replace such as " 0px" with a space so you don't affect ones like 20px, 30px by accident and changing them to a just a 0, which would pass validation. Likewise, you often see 4 separate lines for each border, margin or padding, where you could replace the 4 lines with 1. Multiply that out and I bet you would save a lot of lines of code in a whole theme, multiply that by your total number of themes and look at the potential time and file size saving. More mobile friendly gets you a Google Pagespeed boost too!
Perhaps you could create higher quality versions of some of the original artwork like the snowmen, elves etc, for retina and high DPI devices Like iPads and iPhones. I still use a standard size image for my logos but include a x2 (double size version) which is lovely, gorgeous and crisp on good tablets or 4K monitors.
The extra handy settings for things like Snow in your latest templates is really good, it saves editing templates and is much more elegant, quick and tidy. Nice one!