There are still plenty of dreadfully bad defense teams in t21+ tbh, but t20 to t21 is definitely a big jump, both offense and defense wise. More variation in how much effort is put in by diff players.
I was at t20 recently and my lvl 4 fort managed to win about 90% of matches. Jumped to t21 last week and that number instantly went down to about 40%.
You can even hit t27 with a heavily invested Bike/support/merged mythics team alone if your offense goes very well…but most defenses worth their salt are going to build to counter Bike specifically as one of their first priorities since they are so common. If Bike’s support partner is Corrin and not Lucina, it will definitely be a lot harder because of how crucial the faster aether charge is for Bike, unless Corrin has infantry breath. Corrin is his best budget option, but a Lucina with any ivs is a vast improvement.