if democrats gain senate majority then the alt left extremeists in the white house will aggressively push policies and try to do as much shit as possible to secure as much power as possible (e.g open the borders, let the illegals vote),
I understand what you're getting at but at the same time similar concerns were voiced after Obama beat Romney in 2012.
The thing different this time around though admittedly is that the democratic president can easily go right around the senate and house and use executive orders to get legislation and actions they want passed through to becoming law.
This is because when Obama did this after constant gridlock by house republicans in 2013 and 2014 by late 2014 he started touting how he has a pen and a phone and he can work around congress to get his agenda instated. That set a precedent for future presidents to do the same if they choose.
And to help with fudging the interpretation of the constitution suited to fit these mandates they can just pack the judiciary with activist judges that will interpret the constitution in their favor.
The legislature doesn't have enough power these days and is a paper tiger compared to the expanded powers of the executive and judicial branches of government.
When for example is the last time the president sought permission from congress to go to war instead of unilaterally declaring it (hint didn't happen with Libya in 2011).
this will ensure another republican never gets elected again
Well in the mean time I would advise that if you wanted to try and slow that from happening that you don't vote for any identity politics, open borders candidates and participate in the voting process and vote for candidates that aren't pro identity politics, amnesty or DACA and not pro abortion or chidcare.
That's the least you can do as a citizen of the US and it is your right after all to make your voice heard through your vote.