It appears you believe it was a mistake for Dems outside of that GA district to outspend the GOP by over a 5 to 1 ratio in an election where the odds of winning were slim to none.
To me this Dem party leadership decision is another example of Dem arrogance as it relates to all-things-Trump. E.g. Clinton NOT visiting Wisconsin because her team believe that was in the bag.
Trump is clearly bringing down the GOP and there is much upside for the Dems as it relates to the 2018 mid-terms (but mostly in the House). But as we have discussed it will still be difficult for the Dems to gain the necessary House seat. Many victory margins will be razor thin. The Dems can't afford many more strategic blunders if they wish to prevail regardless of how poorly Trump is viewed or how low his approval ratings go leading up to November 2018.
Losing by less in 2018 than they did in previous elections is NOT prevailing. (something you know but I'm not sure the Dem leadership does after those silly statements by Pelosi).
(PS: as for voter turnout; Dems should be able to get the best voter turnout in a midterm in election history, if they don't that is another example of Dem party leadership failure).
It was a mistake for the Dems to seriously believe they were going to have a victory. It was right for them to compete, but they appeared too sure of themselves. Of course that could have been political hyperbole. But in essence they wasted Millions they do not have in GA.
As for the ratio, I'm not sure it was actually 5 to 1. When you add in all the other groups and private donors and in-kind, etc., probably closer to 50/50 or 60/40.
Another aspect is that it indicates the 2018 races will probably not be about Trump, at least not as much as Dem leadership believes.
As you indicated, if a seat has been held by a very conservative Republican for last 10-20 years, not likely the voters will change to a Democrat even if they do not like Trump. Of course that does not take into consideration that Trump could really do some stupid things and drag the GOP down.
But these districts and some states have been solidly Republican for decades. As you said, the Dems have got to quit making mistakes. The Dem candidates must be honestly seen as reflections of the interests of their states and districts. Not reflective of national Dems, VT, MA or CA. (No offense, but some here seem to think people from those states are future of Dem party.)
As for tunrnout, that is what put Trump into office. The Dems have historically been weak at getting the vote out except in presidential elections. That is how GOP took over House in 2010-Dem voters stayed home; GOP voters didn't.
Not only do the Dems have to gain 24 House seats and several Senate seats, they have to keep the ones they have in 2018 and 2020. That may be difficult.