So, as for the stupidity of Dündar Bey:
I have arrived at the point where he sells Hanli Bazar, and people on Reddit have argued that Dündar is either naive or that there was really no other solution. I beg to differ.
Things Dündar could have done:
get more gold from Ertuğrul's gold mine, and use that if money is what they needed
ask the Sultan for help instead of going to Ares and Saadettin???
band together with the other Turk tribes which they had just made a united peace and trading agreement with???
set a trap to at least figure out who was raiding the caravans???
transfer power to Artuk Bey
set up Aslihan's marriage as soon as Turgut returned so the Kayi and Çavdar tribes can converge and stand stronger
at least think solution-oriented instead of quitting everything immediately
Like, this is what I'm getting off the top of my head just from having watched this show for quite some time now. Dündar is supposed to have been under Ertuğrul's mind and leadership for decades. The thinking process should have bled SOMEWHERE.
Hayme Ana was correct to point out that Dündar Bey is no Ertuğrul. That in and of itself is valid and okay. But Dündar didn't even ask himself what his brother would do. Years they've spent on campaign in the Western region now and Dündar has somehow not understood the very basics of what they are doing there, who their enemies are and what their goal is.
Some people said that Dündar may be a little slow because he got hit on the head as a kid which put him in a coma. And it's true that the Turks didn't seem big on realizing mental health issues even back in season 1 where they almost rather killed Turgut than acknowledge that he was in severe drug withdrawal. So Dündar's so-called stupidity may have completely flown over their head, as he derives his position and authority less from his own deeds and more from the fact that he's the son and brother of someone influential.
I propose a different theory, which is that Dündar has PRIDE and he messed up in the past so he wants to prove himself, choosing a different path than Ertuğrul simply because it's different from what his brother did. If Ertuğrul avoided Saadettin, Dündar is going to work with the Emir, etc. just to prove that he can do it too. He is also too proud to ask for help or to give up his power within his own tribe.
Unfortunately, little Dündar, the scope of the situation has kinda fully escaped you and you are going on a decision spree that destroys not only you and your tribe, but also the other tribes in the region, extended local politics and, by association, your entire state. If you're not a leader (and you are not), are clearly aware of it and are clearly struggling, just give the responsibility to someone else.
Dündar, my baby, you are cute and I've always loved you, but what are you doing. Oof.