Robust diplomacy alongside the ongoing war could be extremely powerful. It is worth remembering that Russia's argument for annexing Crimea was that the population is Russian
And in the not-far-distant past Russia stated that Crimea was Ukrainian.
... half the Ukrainian population (including many who want Ukrainian independence) speak Russian as their first language
Yes, and they were fine with that. It was a happily bi-lingual land. Until Russia created strife in the Donbas after invading Crimea. You're aware that a LOT of the Ukrainian volunteers have Russian as their first language and still fight the invaders?
You saw the huge mass protests in Kherson, a Russian-speaking city, against the occupation?
The inhabitants of Ukraine may have spoken Russian. They sure didnt want to be invaded.
But through diplomacy it might be possible to find a way of persuading Russia they can leave with their integrity sufficiently intact for it not to be a total humiliation in the eyes of their people. E.g. they secure the place of the Russian language in education (Russian language schools were banned)
very, very recently. In the last 3 weeks. This is a distortion of facts and dishonest.
It might not work but failure to try is a lost opportunity.
To even bother trying, you'd have to have faith in Russia's ability to keep its promises. It promised not to invade Ukraine not two weeks before ... it did.
This information is freely available.