In 2019, we spent 9 nights in Padua with only two day trips out of the city --- we loved the city, decided we'd love to live there. Plenty to do and see.
However, specifically about the Scrovegni chapel --- it really depends on how much time you want to spend inside the chapel. It's been my favorite work of art for the last 54 years and I did first go to see it that long ago, when you could spend as much time as you wanted inside. Now, you have only 15 minutes unless you do the double nighttime visit --- in 2019 we did both, so got a total of about 50 minutes.
So, if you really want to see everything Giotto painted (even if you are familiar with much of it from books or youtube videos), you kind of have to take the double night tour. I'm pretty sure that even if you booked back-to-back daytime tickets, you would have to leave the chapel after your 15 minutes, line up to go inside again, sit through the video and dehumidifying again (another 15 minutes), and only then get your second 15 minutes inside.
You only need to stay one night in Padua to do the night double tickets plus one daytime ticket, if that's what you want to do.
Two more thoughts. There will be mosquitoes when you are outside waiting to go in. And I decided to save time inside the chapel by not trying to take photos of the frescoes since there are lots of good (actually, better) photos online and in books.
It's normal for Italian cultural sites not to have tickets online far ahead of time.