Crossing a border by bus without a PCR

Hi! I'm planning a journey to Belgrade for 2 days in the end of September. I am fully vaccinated in Poland however Serbia doesn't accept Polish certificates and demands a PCR test to show on the border. I wanted to get there by bus, so I was wondering if they let me in if I tell them I'm just transiting further? Are the controls very serious?

I can't stand the fact, that Serbian politicians are playing some kind of game, accepting the same certificates from one country, but deny from another...

Anyway I count on your advise!