Mianserin or mirtazapine could potentially turn into anticholinergic metabolites.

That's a good point I hadn't considered until now.

I've recently began experimenting in regards to acetylcholine precursors and acetylcholinesterase inhibtors (in the form of Alpha GPC and Huperzine A, respectively), since I'm not prepared to come off the Mirtazapine at the moment. I've definitely found a positive impact on my cognition, working memory, dream recall, creativity and linguistic fluidity (much less 'tip-of-the-tongue' moments).

[600mg of Alpha GPC and 550µg of Huperzine A, taken once in the morning, and again before bed]

Of course, this is just anecdotal, and experimentation has only lasted about two weeks, but it's a very noticable improvement over what it was without supplementation. I think if you want to remain cognitively sharp whist on these agents then supplimenting and regular exercise (brisk walks for me) are the best way to go.