One. The Republicans still control two of the three branches of the federal government, the executive and judicial branches. Having both houses of Congress is huge. But it is not a magic wand that lets Democrats call all the shots.
Two. The Democrats are not popular. They are at ~43% popularity.
Three. Running Congress for two years and then giving it back cannot happen. This hold on power must be consolidated. It is possible to maintain control of these houses for a long time. Democrats did it for decades.
Democrats ran the House for 40 consecutive years, from 1955 through 1995. Democrats ran the Senate 26 consecutive years, from 1955 to 1981. To hold on to power, Democrats cannot overreach as Trump did, they must focus on policies and tactics that do not give back control again. That means being honest about what policies have public support and which don’t. That means not picking fights that just lead to these new Congresspeople from red states being dumped next chance.
Four. The GOP was, and is, a minority party. They hold on to power through various anti-democratic means.
Conclusion. The policies to prioritize are those that allow elections to reflect the will of the people. This will “lock in” the majority that the Democrats already have, as long as they don’t focus on unpopular policies and strategies.