He didn't struggle to damage them though? He easily withstood a sword slash directly to his arm, then proceeded to easily cut into Mahoraga's skin and caused bleeding, putting Mahoraga on his feet in pain:
The sword slash that Sukuna withstood was from a sword enveloped in positive energy, when the blade was instead
imbued with cursed energy,
it sent Sukuna flying outside of the Curtain.
Then it adapted to be able to deflect the attack. So Sukuna instead used the attack while touching him, and it left a large gash on his front side and laid him out until he regenerated from the wound:
Sukuna's casual Dismantles can
cleanly slice off body parts of Special Grade Curses and
turn people into cubes yet
Mahoraga only suffers surface cuts from Dismantle indicating his durability is great enough to contend with Sukuna's physicals as Sukuna's slashes deal piercing damage and Cursed Techniques have higher output than physicals.
If it is, I don't think it'd be by any relevant margin. Cursed Techniques work by applying Cursed Energy like electricity into an appliance. The same Cursed Energy can also just instead be used for raw physical attacks as demonstrated by Gojo:
The same "electricity" (Cursed Energy) that can be applied to Electronics (Cursed Techniques) can be applied to physical attacks. Basically, if Dismantle cut through a building, Sukuna applied enough Cursed Energy to supply a building cutting attack. He would thus then be able to apply that same energy to his punches/kicks.
Of course, in the case of techniques that completely ignore durability and have no way to be calc'd (I.e. Idle Transfiguration), then yeah, it'd be hard to translate that to raw AP. But for something simple like a cutting attack, it should be more than fine. Same for the fire arrow.
This is quite literally the issue this CRT is tackling (assuming special techniques scale to physicals due to UES, which kind of defeats the point of using a special technique to bring out power), in fact we're told in the series that
Ryu is the only character who can output the same amount of Cursed Energy regardless of if he is using his Cursed Technique.
While testing Mahoraga's capabilities, he left wounds deep enough to cause Mahoraga to gush blood and fall onto the ground in pain (Temporarily incapacitated) until he regenerated and got back up. And even after adapting to the cuts and becoming more resistant to it, Domain Expansions cuts managed to shatter the blades on his hands. Then when he got serious about finishing Mahoraga off, he used the flame arrow. Just seems like he wasn't intended to finish him off until after testing his adaptation.
This doesn't change the fact that Sukuna's casual Dismantles can completely slice through Special Grade Curses meanwhile Sukuna empowered his Dismantles more against Mahoraga as seen by him
using the hand signs of his technique (which canonically make techniques stronger) yet it only deals surface cuts, that alone shows that Mahoraga's durability is at least somewhere near the ballpark of Sukuna's physicals.
To me it seemed quite clear he was aware it possessed some talent that would be useful for Sukuna later. This had been foreshadowed with him believing Fushiguro could beat a finger-bearer and how it was later used against Gojo who's techniques he was aware of.
He even said "If I'm understanding this correctly, it'll stand up". Only to be proven right and ecstatic when his assumption was correct. Mahoraga had been showing to adapt since the beginning of the fight too as noted by White above.
KingTempest has already debunked this in the post after this on the thread.