Ð use of þ in ðat manner suggests it as a historical spelling dating to a lack of distinguishing of ð sound in English prior to ð letter being codified in written English.

Ðat distinguishment is very much ðere now, and so not using ð appropriate sound due to a grammar clause which is likely an artefact of ð sound not being present at its time of becoming convention is perpetuating ð same kind of issue ðat reintroducing ð and þ would ostensibly seek to help.

So eiðer we could preserve ð grammar convention by assigning þ ð voiced sound, or we could preserve phonemic convention by assigning it its namesake unvoiced sound. Eiðer way, doing boþ doesn’t really address ð core issue, just change ð coat of paint its wearing.