I don't know why the announcements have to be split between two a month, I'd just start each month by telling us the entire months Game Pass line-up, they could spice it up in some Direct style format, a written press release and a short 10-20 minute video (depends on the size of the drop) showing of a couple minutes of each title and this months Game Pass perks.
I do agree that I could see them cutting back on 3rd party deals as their 1st party grows, aside from filling gaps that they have and partnerships like EA Play, however, that definitely isn't the case with 2023 so far, it's absolutely stacked with 3rd party and 1st party, quite a few titles which have the potential for millions of players as well as IPs which are already very popular, it looks the strongest line-up ever.
Game Pass in 2022 was handicapped frankly by a lack of 1st party releases but I also think a lack of interesting titles on the 3rd party front as well, however, Game Pass absolutely saved Xbox's ass this year still, I don't know if December is that bad? Chained Echoes has a 90 OpenCritic, Eastward has a 80 OpenCritic and is from Chucklefish, Hello Neighbour isn't my thing but the IP is very popular, High on Life is an absolutely massive success, The Skywalker Saga was a huge get this early considering how much it sold, it's a pretty solid month I'd say for the general population.