"Folio: is dedicated to providing magazine publishing professionals with the news, insights, and best practices to keep them in tune with today’s media industry trends."

"MPA – The Association of Magazine Media is the primary advocate and voice for the magazine media industry, driving thought leadership and game‐changing strategies to promote the industry’s vitality, increase its revenues and grow market share."
Lots of good info - look under Research * Resources

Poke around this site - lots of interesting things. Not all are free.

"If someone were to tell you that the answers to your most pressing questions about building digital magazines, were already available, would you take the time to read them?
The information in the 2018 Mequoda Magazine Consumer Study will put you on a path to creating a multiplatform magazine that enlightens your readers and helps your publishing company grow and profit."
very brief synopsis at https://www.marketingcharts.com/cross-media-and-traditional/magazines-traditional-and-cross-channel-83731

Not always up to date, but has some nice stuff, with links to studies, etc (older stuff can be useful too, to show trends/growth). Try a search for Digital Magazines to start.

Buried are some reports on book publishing and digital magazines. They are old (~2008), but might be a starting point for analysis.