This manga is 27 years old and you still didn't notice it?... power levels in One Piece are inconstant, people. Shiryu will be as strong as he needs to be.
If Oda needs Mr. 5's and Mr. 3's DF powers to be good enough to capture 2 of the most powerful Elbaf giants, who now have an updated bounty of 1.8 bil each, he'll have it done (already happened). If Oda needs pre-skip Smoker to be strong enough to stall Ace, despite not being able to beat Vergo post-skip, he'll be (already happened). If Oda needs pre-skip Crocodile to be strong enough to clash against New World veterans with well-developed haki, despite losing to Arabasta Luffy, he'll be (already happened). And so on...