Hey Steph, so I had this same issue the first few weeks with Lilith, and I have a few general tips.
1) Don’t pick your bunny up to go in the cage. I did this with Lilith a few times, and I found it really hurt our bond early on (and I am slightly paying for it now). You do not want her associating being picked up or handled with being caged. Same for treats, you dont want her to learn treats mean cage time, because she likely will wisen up haha.
2) Plan for bedtime early. This is what I now do. Instead of a firm bedtime like say 11:00PM, I now start viewing bedtime as 1030-1130 or 10:00-11:00. What this allows me to do is wait for Lilith to go into her cage on her own. After she goes in I merely walk up and close it, simple as that. This does require all of his/her food to be in the cage, so the bunny needs to go in. This way you arent forcing them in, and Lilith now is fine with it. I will say after you lock it, quickly give your bunny a treat haha. Can’t have him/her mad now can we .
3) Instead of bribing with treats to get OUT of the bed, put the treats in the cage before hand to encourage her to go in there. Many forum users when I was researching this issue suggesting finding one treat that the bunny likes that is ONLY given at bedtime in the cage. For many users it seems to work. When my rabbit gets older and is allowed to have more treats, I plan to associate bananas with bed time. So when its bed time, she runs to the cage for her night time banana piece. Make take a while for the association but it seems to work.
4) The cage may be big enough, but another suggestion many people said worked is getting a bigger cage. Bunnies love space, and the more the better. So eventually you could look at building a solid NIC cage (you could easily do it for 70-100 bucks). I know its expensive for younger people, but considering how expensive some of those normal bought cages are, its not bad.
5) If you dont want a new cage, try buying a dog exercise pen! This is a great idea for not locking your bunny up, but your still ensuring he cant get out. Get the taller one 36 inches plus, but it works so well. You can then attach the pen to the front of the cage and give the bunny a front yard. I cant trust lilith yet, but she has like a 4×4 enclosure outside her NIC cage she is free to run around in at night that has no access to chewing floors or what not. But at night you close the pen but keep the cage door open, and bunny is happy and your happy. Then in the day time you simply open the dog pen up for her to have her full run time. This would also ensure your bunny is getting all the exercise and helps you see how the bunny handles being out of her cage for extended periods (litter box habits, etc.).
Hope this helped, good luck!