Here's an idea that won't require you to "guard" anything, but will help in defenses for sure.
Historically with radar ranges and advance ground spotters, NO big bombing raid would go unnoticed. Defensive fighters could have more than enough notice to scramble, climb, and meet the bombers at alt in force.
Both sides could tell when large missions were forming.
In this game we barely have 5 minutes' warning (the time it takes for a bomber to cross a sector).
I would suggest the gameplay would change if ALL enemy bombers show up on dar if they pass 15k alt. Unlimited dot dar ANYwhere on the map, for bomber formations above 15k.
That would give us enough ready-alert time to scramble and climb out to meet them.
Otherwise, bombers fly too high, too fast, and too undetected. They are the stealth bombers of the modern era, only without the miniscule bombload.