I'm sorry, but I just can't see Bartylus as toxic. They were already best friends before they got together, and Barty is very protective of his friends, so you already know he 100% fought multiple people for saying stuff about Regulus. Regulus was also protective but in a more silent way. Lots of people already talk a lot of shit about Barty because of the way he acts, but Regulus would put potions in some of those people's drinks and it would make them go bald or break out in hives or something. Also, they would absolutely be there for each other a lot. Regulus would help Barty a lot after his mom passed, and Barty would be there for Regulus when Sirius got disowned. Overall, I just can't see them as toxic, I see them more as scarily devoted to the other and would do anything for each other, and I think that's beautiful.