A long time ago I used much the same solution (when using the Vienna RSS reader, but the same issue) but in the end just decided not to open links in a Chromium-based browser, because I found the constant switching from browser to RSS reader to be slow and annoying.
Personally I think it is better to use an app such as Choosy to set links from the RSS reader to open in a browser that respects "open in background" settings – and then of course you can leave the Chromium browser as your default for everything else.
To each their own, I suppose. I personally use browsers for different contexts, and having the RSS links open in the proper context is worth it to me. On principle I agree that Chromium should "respect" this setting, but I can't punish the browser by using another one (in this case I would only be punishing myself).
Also, I should mention that on an M1 Mac the switching animation may still be annoying (a quick flash), but it's almost imperceptibly fast.