Sam2- My very erudite son informed me that the Shach- in his major work on Sefeikos,end of joreh deah- paskens like the Rashbo that “sefeika d’oraisa” is lechumroh- and does not pasken like the Rambam who, holds it is only lechumro miderabbonon.
However, as far as a psak when the doubt is on matters of which halacha to follow- there are multiple variables and “klollim” and it is impossbile to say whether we pasken “lechumoroh” or ‘lekuloh” -regardless whether it is midoirasa or miderbbonon. I doubt there is one size fits all.