This feels like it should be nearing the end, but has been put on hiatus for a spin-off? =/
Rough...
@konzolmester
If a story's main lead is a girl and has romance tag in it then its either shoujo, josei, or neutral.
Go read
Claymore.
There isn't much romance, but there is a little. The main character is female. And it is one of the most blatantly Seinen series around.
Should serve as a perfect disproof of your erronous notions.
Who is or isn't the main character doesn't dictate the demographic for a manga (especially when they're not the viewpoint character). Genres may by biased for one demographic and against others, but those don't dictate it either.
Rather it is how the characters are designed and how their interactions are portrayed that are the giveaway for whether something is shounen, shoujo, seinen or josei.
For THIS story, the seinen tag is justified.
Shounen romance is overly simplistic, mostly based on the male adopting a protective role (often of girls indiscriminately)... but being a bumbling idiot most of the time, and the authority in the romance is in the hands of the girls.
Shoujo romance is likewise overly simplistic, mostly based on the female caring for men (again, often indiscriminately)... but being a "delicate flower" type character that all the tall, gangly men try to sexually dominate, so again they have the authority in the romance.
Seinen and Josei relationships are generally portrayed with a bit more realism, generally fewer false harems and contrived situations, but regardless of who is or isn't the "main character", the viewpoint character for seinen is usually male and the viewpoint character for josei is usually female... while the romance authority is in the hands of the female for seinen series and in the hands of the male for josei series. And as usual, the relationships mostly show progress when the authority dynamic temporarily reverses.
In case you hadn't noticed... for the interactions between Takamura and Senko in this series, Senko is effectively dominating. Takamura is the viewpoint character. Ergo this could only be Shounen or Seinen... and it lacks the obviously contrived nonsense and school-child idealism common to Shounen plots. Ergo this is Seinen.
If you want to see a blatant Josei marked as Seinen...
Kusuriya no Hitorigoto would be the example I'd pick. That series couldn't possibly be more josei if it tried.
At least as far as romance is concerned, if you want to know the real demographic.... just look to see who is being romantically passive most of the time... Their prospective love-interests will have exaggerated personalities to draw appeal to themselves, but the one meant to be a viewpoint character will generally be (at least from a romantic perspective) generic and uninteresting
by design as they're made that way for the reader to... well... substitute themselves into the role without conflict.