I would have to disagree on much of what you mentioned, we lack huge territories but that does not mean that ipso facto there are no ways to defend or that we'd naturally be an easy target, we're still bigger and have more depth than Israel, more than Lebanon, more than Switzerland, more than Greece, bigger than North and South Korea or Cuba and roughly the size of Syria and Britain (bigger than England still.)
so after all we do have enough depth to hold back an invasion or build up an attack (ofc we're talking here ignoring both the economy and status quo)
our topography isn't that easy either, most of our western lands are mountains and hills thanks to the dorsal and Tell chains with the peak at 1544 m and the other mountains at an average of +1000 m
nevertheless the dense forests and vegetation from the Khmir mountains down to between Gafsa and Sidi BouZid at least.
if you ever visited these regions you will notice how easy it is actually to relatively pass from one mountain to another and till you find yourself in the Atlas mountains, and that these regions provide good food, water and refuge ( thus the reason terrorists weren't 100% eradicated till now)
then you have the desert, mostly hostile terrain but also mountainous in the south east as it gets closer to Nafousa mountains in Libya, the few plains we have are in the Sahel, Sfax, central kairouane and around the capital and the Cap Bon.
now for what we need most, historically Tunisia's golden age was always when we had a strong navy: during the times of Carthage with the Aghalbids when we annexed Sicily and during the Husaynid dynasty (until we lost most of it in Navarin in a war that wasn't even ours).
so if we want to have the initiative we need a huge navy with couple of capable submarines, I still think submarines are the best weapon a nation that lacks massive territory and manpower can use to get some balance of terror (Singapore a good example of that ) and we need an air force that can reach deep inside others' territory and proceed with preemptive strikes when necessary.
well ofc this only remains mere hypothesis for fun, unless either we decide to be like North Korea and spend 23% of our GDP on army instead of the current 1.9%