Belarus is rather unimportant. They are 10 million country with GDP per capita lower than Russia, spending little bit more than 1% of GDP on their army. Belarusian army has also zero combat experience. They have no reason whatsoever to die in Putin's war. No glory, no territory that they want, no hatered of Ukrainians. There were soldiers openly calling for desertion which was rather impressive at least for me. It's possible that Lukashenko was actively trying to join the conflict, but the army told him no.

Poland was offering something of the form of peacekeeping mission and Zelensky was afraid that it could lead to freezing of the conflict based on his own reporting in an interview with Meduza. Ukrainians wanted to go on counteroffensive from the beginning.