>Through Belarus
Right this is the border between Belarus and Ukraine. What immediately leaps out to you about it? One there's a real lack of decent roads, two there's lots of forested areas and trees and three there's lots of specs of blue.
What that should tell you is that that is very boggy, waterlogged ground with lots of positions open for ambush that your columns are not going to be able to bypass easily. It's highly defensible ground where attackers can be easily channelled into killboxes.
If you remember the early invasion you'll remember the 20 mile traffic jam to Kyiv. It would be like that, all over again wherever you go. Only this time Ukraine has shit like ATACMs to really fuck over a stalled out column.

>a amphibious landing from the sea in the south?
The black sea fleet doesn't have the capability to make an opposed landing and the Russian air force doesn't have the capability to secure air superiority, let alone supremacy. They've lost several assault ships, which seem to have been prioritised in the early war, probably to prevent any attempt at landing.

>Or even another Air Assault?
See above, re. air supremacy.