Yeah, it’s really common for indie games to be derivative of big name games. Examples:

Wargroove (advanced wars)

Stardew Valley (harvest moon)

Etc. Wargroove isn’t really even “indie” as Chucklefish is a very established studio.

So, yeah… this is normal and not worthy of criticism.

A genre containing only one game series from one company? :P

There are probably 100s of this style of game... and from many companies. I'm not exaggerating. Many before "advance wars", too.

The earliest one I've played was Operation Combat on the Amiga in 1991.

https://youtu.be/uC845vxdyuY

Hadn't heard of it --cool. Along the same lines, Eastern Front[0] was a turn based tactical game about 10 years earlier. I played it in the early 90s on an Atari. The OPs game and advance wars have much more in common, but certainly the derivation goes back a long way. Later on, Japan had a lot of these turn based tacticals that didn't make it to western markets. (Super Robot Wars is a series that had some titles sold in English but not most.)

[0] https://en.wikipedia.org/wiki/Eastern_Front_(1941)

If there are so many, can you show one example at least?