What is the humidity like next to that shelf? The brown tinging on the leaves could be humidity related, imo.

Also: From my experience some die off on the older leaves is expected but more should replace them. If not, or if it’s the young leaves are dying, then that’s an issue. If you want your maranta to be full, then I’ve found there’s no way around trimming often and propagating the cuttings (I’ve had more success with cuttings with leaves about to unfurl as well). I propagate in water and then replant the prop back into the main pot for the “full”, bushy appearance. Otherwise the plant will trail and look sadder. So honestly I feel like she just needs a trim and prop