You need an XL or Pro XL cruiser to race. Youd feel like a monkey trying to hug a football on a 20” bike
by getting into BMX, are you talking about racing? Skatepark/dirt jump stuff or just riding street?
i just picked up a 29” SE Big Flyer a couple of weeks ago to ride at our local pump track and general screwing around on. It iss a great bike for that purpose and would fit you well at a low price
SE has a lot of cruiser options