Can a Belgian citizen automatically receive a new 30 days upon exiting and re-entering Thailand during a visa-free period?

Depending on where you are in thailand AND what border you think you're going to cross and return from I'd say it's cheaper for you to go to the immigration office in your area and take the 30 day extension for 1900baht.

Take the 30 day extension and if you need more time after that then think about border bouncing, but pick the border carefully OR use a service to bounce you out and back so you don't get hung up on the border comin' back in.

There is no "how long should I spend outside" you aren't fooling anyone at passport control, they'll know you only left the country so you could milk a new free stamp entry, so just use a service to bounce you out and back. That way you'll skate right back in