There can be potential wargame analysis done if say the force is sufficiently far enough away at the start (say Indochina somewhere), allowing Singapore to attack it all the way down the kra isthmus and malayan peninsula etc....to properly leverage its airforce capability and so on. i.e some kind of modern day hypothetical w.r.t Japanese operations in the area WW2.
Then it depends on details you fill in regarding that. But if they have reached Johor its game over for Singapore, it simply has run out of room at that point. Not even going to go into what it would have to had stockpiled for the earlier phase war of this force (gathering and making its way down).
Heck, the British Army did in fact resisted starting all the way up to Kra Isthmus. Doesn't really help them that much, no difference, they keep retreating to Singapore. The British has decent Navy back then, based in Singapore, yet they failed to intercept Japanese landing, unable to beat back Japanese Navy, and HMS Prince of Wales and Repulse were sunk.
Singapore best chance is to dig in and prepare for a long siege. 'Decisive military operation' is no more than a fantasy, Russia tried to do such a thing, by mounting assault to Kyiv and other citiies, with a force much larger than anything Singapore can muster, yet still failed horribly.
So they have two options:
1. Their current wishful thinking of 'preemptive strike', whatever that BS is. Which requires
no mistake in planning, and execution. Because anything less than that will allow the opposing army to retain its strength and cohesion. Once failed, mounting counterattack will be next to impossible as they will lose momentum. Russia tried to do this early in the war, but faulty planning and execution made them failed to achieve their objective, and lose momentum, unable to assault Kyiv again.
2. Dig in and entrench Singapore. This is the more sound strategy, as they can have a more certainty building defenses in their own small territory, instead of guesswork in a 'fog of war' (unless they seriously believe they know everything and has no disadvantage). They can methodically prepare to resist any attempt to assault Singapore, and can more easily prepare their forces and order of battle in their own ground. This is why Ukraine was able to resist for so long, Kyiv, Mariupol, and Bakhmut are some of the battles where defensive strategy allow Ukraine to resist Russia effectively, with a sound management of time, space, and resources.
Choosing to do a field battle for an inexperienced army is not a good option, let alone expecting an easy win.