For me, the best tactics in this regard will be to trade grounds and absord the attack as deep as possible, avoid head-onn battle early on and deny the advancing enemy a chance for a decisive engagement in their favor. at the same time, using highly mobile hit and run tactics, continuosly harassing enemy logistic tail. Either the enemy will slow down their attack to cope up with reduce logistics (as a result of the attacks) or they'll outrun their logistics. once this happen, i will release the main force with an objective to surround and cutoff the enemy deep inside our territory.
The 11th KAD is not in proximity, but there are other units who are supposed to stop an invasion.
Northern Malaysian peninsular is protected by the Army's 3rd Division, which is the best equipped Division of the Malaysian army except for it's lack of MBT's.
It consists (combat units only) of 3 brigades, among them the 4th Mechanized, the only Mechanized Malaysian brigade (ACV-300's and MIFV's).
All in all, the 3rd Division brings up:
6 Light Infantry Battalions
2 Light Infantry Reserve Battalions
1 Parachute Reserve Battalion
3 Mechanized Infantry Battalions
1 Mechanized Infantry Reserve Battalion
2 Armoured Battalions (with Sibmas-90 AFSV)
5 Artillery Battalions
3rd Division has the only Malaysian Mechanized Brigade, it's the only Division to have two Armoured Battalions (one of them, the 1st KAD, is directly associated to the 4th Mechanized) and it has half of Malaysia's whole Artillery.
I don't think that such a force would be based in the north if the Generals' plan would be to let the Thai army advance far into the country at first, before striking back. And also, I don't think that the whole defence plan is centered around the the 48 PT-91M's. Don't forget, these tanks are completely new, not even fully in service yet, whereas Thailand has it's overwhelming tankfleet since decades. There must have been a defence plan before the PT-91M's had arrived, and I guess it still works today, with some punch added by the MBT's.