We know exactly how the war would go without the external support because that's how the war started. Russia picked the time, date, place and had the initiative, Ukraine wasn't convinced that Russia was going to invade despite US warnings, and yet despite all that the Russian advance bogged down in three days. After one month multiple intelligence sources and an accidental leak from Russia said Russia had experienced around 10,000 fatalities. This was during the period before significant amounts of arms had arrived from other countries. This was done by Ukraine largely using what it had at the outbreak of war. That is a huge number of dead in such a short time. This is not Russia showing competence.
Like you said, Russia hasn't mobilised, but they wouldn't start a war they thought they wouldn't win so the fact they haven't mobilised and invaded anyway shows poor intelligence and lack of judgement. The fact Russia didn't mobilise isn't evidence of Russia secretly been good actually, it's more evidence of incompetence.
And clearly Russia hasn't sent its entire military but since their military is much larger than Ukraine's they can pick the units and equipment most suitable for the mission, where Ukraine just has to use everything, regular military, reservists, paramilitaries, and in many cases volunteers with literally hours of training. So even if the numbers are similar the advantage would still be with Russia.
No matter how you try to paint it Russia had nearly every advantage yet still managed to largely stall in just three days and experience huge losses doing it. It is blindingly clear the Russian military is doing a very incompetent job and it takes some very motivated reasoning to begin to argue otherwise.
It's not just enough to call for critical thinking, you actually have to practice it.