I agree they are moving too fast. I quit playing for 3 months only to find out I had completely missed Mirkwood, and they were at Isengard already..... I'm now just catching up, and it is a lot of work! haha!
I would suggest to go to 6 months intervals with this timeline:
March 2020: Rohan/Wildermore (85)
September 2020: Helms Deep/Fangorn (95)
March 2021: West & Central Gondor (100) - do NOT release ALL level 100 content at the same time.
September 2021: East Gondor & Old Anorien (100)
March 2022: Far Anorien & March of Kings (105)
September 2022: Mordor & Dale (115)
What comes after..... I will let go for now, as there are a fair amount of options.
But to my taste, they are moving a bit too fast.
I also would not mind a 3-month release with actual content releases, e.g. March 2020 will be Rohan, and then 3 months later June 2020 will be Wildermore, then in September 2020 Helms deep for level 95, 3 months later Fangorn, and so on, and so on.
So, content releases every 3 months.
Level increases every 6 months.
To me, that is fast enough.