There's no way western militaries would just add troops to the front. If we're fighting a war with Russia - regardless of where it is - it would be no holds barred. And just adding more troops to the front is not the way an advanced tech-heavy loss-averse western military would do it.

The only way western troops are deployed en-mass inside Ukraine is as a peace guarantee with some level of Russian agreement. The moment there's footage of British/French/German troops getting hunted down by drones, the public will be crying for direct strikes into Russia and it'll be WW3.