I sucessfully upgraded tl-wr841 v8 to 16 MB flash. The firmware is now 18.06.1 16MB Build.
With wifi I currently experience DHCP address problems: When connected for the first time, only IP V6 is assigned, the IP V4 not. When I reconnect wifi on client, I get IP V4 for the second time assigned. With ethernet I get immediately IP V4 as well.