The defender should not assume the attacker will come in a squadron transit. They might build a jump gate and punch through the center. Going through the center makes a lot of sense if the attackers plan on transiting, dropping their own minefield or sensor equipped missiles, and transiting out immediately after 30 seconds.
So the defenders absolutely have to dominate a range in which they can kill their enemy in less than 30 seconds.
One option is to heavily mine the jump point so the enemy can't jump OUT safely. Sure, they can pop out 250,000 km from the jump point, but getting out, they would have to run the gauntlet.
Mines would be of a great help to the defense, but if the attacker sends in a single ship to draw fire, a huge number of missiles could be wasted. So having more thermal mines than are necessary to kill a couple of ships would likely be a waste.
But EM homing weapons are a different story. You would need the range to cover transit plus the distance the attackers would go before their active sensors come on line, but as long as the defenders can handle solo ship probes before the actives trigger the EM mines, the EM homing weapons can be effective.
On the defenders advantage, they can have a lot of 'mines' that are simply active sensor drones. Perhaps with armor, or just very, very cheap. If the defender's problem is a tech disadvantage, a huge number of active sensors might just help a bit in getting details of the attacker's technology. Complicates their point defense calculation as well.