That’s slightly more than a month for an enemy. They require concept art, 3D modelling, animations, voice acting, gameplay design, visual and sound effects, lengthy and thorough playtesting, bugfixing…
You think all of this can fit in slightly more than a month? This process takes multiple months, half a year or more, for a single enemy. And the staff members you need to do this are also working on animations for new weapons, or voice direction for the NPCs and player characters, SFX for new maps, etc.
Fatshark has about 180 employees total. You can expect fully half of those to be non-development roles like HR, project management, community managers, marketing, finance, etc. They self-publish so those jobs aren’t filled by a parent company. So maybe 90 developers and artists and programmers, but maybe even less.
Then you subtract the VT2 team from the remaining developer count, who are smaller than DT as per Aqshy but still sizable.
You aren’t left with nearly enough manpower to make that much content in one year. The team is much smaller than you think. And Swedish salaries are expensive, so don’t think they can expand even more than they already have based on the Tide game revenue.
People expect a live service or a lifestyle game but that’s not what Fatshark can deliver. They have quarterly updates of varying sizes and content density, and the playercount ebbing between them is only natural.
EDIT: that said, I wouldn’t be surprised if Fatshark could be way more efficient with their development process than they currently are. But that’s beyond my ken. Just looking at the numbers game and having been part of playtesting, I understand why the content pace is the way it is.