Yeah but keep in mind Estarosssa strength stat which is a measure of his physical capabilities is extremely high. Yet Escanor proved that his 11:30 self was more than enough to outclass and outspeed it's 50,000ish value. I wouldn't be surprised if prime Meliodas's strength was under Estarosssa's strength stat or barely above it. If Meliodas had physical capabilities (strength stat) of let's say 60,000; I believe that 11:30 Escanor would certainly give him a challenge and that peak Escanor would be too much for him to track. Escanor would still have half an hour left for his powerlevels to rise right after a big surge, and then they'd most likely surge again at his peak.
What we know so far(strictly in terms of PL)
Estarossa(Magic: 3000 Strength: 53000 Spirit: 4000 Total: 60000)
Zeldris( Magic: 10000 Strength: 47200 Spirit 3800 Total: 61000)
Prime Meliodas(I assume he is at is prime) (Magic: Unknown Strength: Unknown Spirit: Unknown Total: 60000)
Even assuming that Meliodas is inferior to Estarossa in terms of Strength(which is at least above Derrierie's 48K). His PL is like that without his mark. If he uses his PoD, considering everything we know so far, do you think Estarossa's PL could be comparable to him?
What I am saying is completely consistent with him being unable to control his power though. You are talking about a guy who does his best 12 hours a day to suppress his power as much as he can. And yet the bare minimum at which his power increases is still thousands worth of PL per battle. The slightest arousal causes his power level to surge. And given what we have seen, rhitta does certainly have the function to hold him back, otherwise his comment at the end of his fight with estarosa makes no sense. Of course, clearly rhitta also has the opposite function of helping him in battle just as much as any other sacred treasure would in battle.
Escanor is not trying to suppress his power though, what he tries to suppress(at times) is the heat he emits. His PL doesn't increase as you said. So far, it only surged during the night against Gowther and at 11:00/11:30 AM against Estarossa. Against Galan it was clearly rising with 5 points per seconds.
Rhitta doesn't hold him back, as I said just above, what he tries to suppress is the heat he emits in order to not burn everything around him, and the heat intensifies at times depending on his emotions. Like in the Special at the beach when he saw Merlin and in his fight against Estarossa. That's what he meant, he was too excited to contain that heat and he said Rhitta would not be enough to contain everything either. Against Gowther, his emotions are what transformed him as well but since it was night his PL was not rising.
Escanor can't not emit heat, it's inevitable, but Rhitta stores that heat so that he can release it after as attack, he can also use his Magic on it(probably the weapon allowing him do that) and it's a mighty weapon fitting for a man of his build. Sacred Treasures have been made to support the Sins in battle and suit them, they are in no way holding them back. Escanor without Rhitta would have less time to help his companions because Rhitta would not be here to store the heat.