Hello I will help you with your question,
Your northstar is certainly a victim of the design flaw that allows the cylinder head bolts to tear out of the block from thermal expansion despite the block test results.
This means you are burning the coolant and the problem has gotten to the point that it can no longer be operated for any length of time on the road.
The acceleration causes the cylinder pressures to raise so the bolts pull out a little and the head gaskets leak. They go back in place at idle so that is why you can idle for hours but not run down the road.
The sealer will not cure this problem, the only fix is to disassemble the engine and install "Timesert" thread repair inserts or as you say "sell it and buy a non northstar engine"
Sorry for the bad news but if you google northstar head gasket problems you will find thousands of posts on this subject
Please let me know what I can do to help you
The timeserts are installed in the holes in the block that the head bolts are threaded into. The cost to do this is more than installing a remanufactured engine like a Jasper (3 yr 100,000 mile warranty) which runs about $7000 installed.
In the case of installing the timeserts the engine should be rebuilt as all the labor is in the disassembly and reassembly so while it is apart you should replace all the wear parts. This can be done in the vehicle by just removing the cylinder heads drilling and installing the inserts and reassembling but you still have a bottom end that may fail and you are installing new gaskets and seals forn the upper. I would not recommend this route if you plan on keeping the car for many more years. You still have the issue of the solenoids in the transmission that overheat and fail so you have to make a decision of how much you are willing to put into this vehicle vs a new car payment or a newer used car.
Once again I am sorry to hand you this news but please spend a little time with google and see how widespread the problem is.
Let me know what I can do to help
From 2004-5 on up there is a better design so after that the engine will not have this problem but you have to understand that GM no longer builds cars/engines to last. They believe that every five years you need to buy a new car and they engineer them to last the length of the warranty. With all the bells and whistles they are installing you have lots of electrical/electronic failures and many of the features don't last as long as designed. The basic (expensive) systems of the vehicles last now but the switch gear, window regulator, backup sensors, navigation systems etc do not do well in the automotive enviorment.
Please let me know if you have further questions about this