Block force has nothing to do with blocking power. It's just how much an attacker gets knocked back.

When you parry you get parry value x block armor. So your buckler actually has more block armor than the tower shield during the duration of the parry. The amount of stamina used blocking is based on the % of block armor used.

You can use a tower shield to turtle but just not against heavy hitting enemies for long. It's unfair to judge the tower shields innate blocking ability against the bucklers small window of strength and in only one scenario.

Parry is probably still op, but they won't be able to tone it down without causing a bunch of drama.