Is there a way in Connect to see average daily number of steps in a given year (eg 2023) instead of the rolling previous 12 months?
Yes, you can see the daily average for the selected year (rounded to the nearest 500) in Garmin Connect Web » Insights:

Please note that the steps insights are limited to 15k daily steps, so if you have more than that, you'll still see just the 15k (just like on the screenshot above)