The only way Thanos might be able to 'solo' is via BFR. Without that cheap crutch to lean on, however, he is definitely not winning on his own. Remember, Superman was massively amped when he fought DD during the H/P arc, but was still effortlessly swatted aside like fodder. DD also owned Darkseid himself as though he was an inept feeb(which, back in the early 90's, was quite unprecedented.) And in DD Wars, he was steamrolling teams of elite high-heralds like they were d-listers.
Point: both H/P DD and Thanos are solid trans-levelers. Thanos would be very hard-pressed just to contend with DD one-on-one, nevermind 'soloing' both he and Dentry simultaneously.