There are several ways to measure schedule difficulty. The most outdated one is the use of win/loss records from the prior season. We can look at how the New York Jets performed, but their offensive numbers would be inaccurate since Aaron Rodgers didn’t play.
The Seattle Seahawks' defense should be night and day compared to last year's defense after hiring Mike MacDonald as their defensive coordinator. You can get into the scheduling quirks, spots, injuries, etc.
2023 record
When you use last year’s record, the 49ers have the 13th-most difficult strength of schedule:
Every team's strength of schedule for the season.