This may sound ridiculous, and even be so, but it is my experience after decades of teaching. I spent most of my life as a very hard nosed grader, totally intolerant of any cheating, regarding it as a violation of university rules as well as the sanctity of my relationship with the students, wherein I was sincerely trying to help them. I wondered how they would like it if I lied to them, thereby cheating them of a chance to learn the material correctly. I usually had only about one cheater in a class of 30 or so, but that one made my life miserable, and absorbed more energy than all the rest.
Finally as I aged, I changed completely, and began to just try to remove the fear the students had of failing, which led them to cheat. When monitoring a test, if I saw a student glance surreptitiousy at a neighbor's paper, instead of marching back and ordering them to move up front where I could supervise them better, I simply walked back with a smile and asked if there was something that was giving them trouble. Usually they indicated a question they had no clue about, and I would just give them a helpful hint. If I saw an error on their paper I might suggest they rethink that one. Then I went back up front and wrote the same hint on the board for everyone.
I tried to give the impression, and to actually feel, that I simply cared about helping them learn the material as well as possible. When they began to buy into this. they seemed to stop trying to fool me, or maybe I just quit caring so much about the few who had been the whole problem in the first place. The result seemed not to be much change in the grades, i.e. the weak students didn't do much better even with the extra help, but they felt better about our relationship. It was no longer me against them but the both of us united trying to master the topic.
So naive as this sounds, under the present circumstances, I would just make very clear to the class just what behavior is acceptable for online tests, and try to make help available, and see how they respond. Maybe extra attempts could be offered in case the online format makes it harder for them to express themselves, some kind of retakes.
I realize my hard nosedness, from which I was finally liberated, is probably much greater than most other peoples, and that most people here already are more sympathetic than I was, so this may not change your approach as much as it did mine. There is also the inevitable annoyance that some answers handed back are not legit, and the scores are changing in a way that has no logical explanation, but it did seem to help me to try to focus more on helping and less on catching people out. The future is not kind to the cheater ultimately, as they don't learn anything, and that becomes clear under any kind of real test. I learned to avoid seeking to punish the person who tried to cheat, and made an effort to teach them this is a poor choice. Of course I tried also not to reward it.
I also have struggled greatly with the small amount of online teaching I have done, which was only with extremely gifted youngsters. In my experience it seemed the child took advantage of the distance between us to mostly sit back, and let me do all the work. It became more of a performance by me, than a shared lesson, and I felt the child learned little, even when they enjoyed and appreciated it. In an in person class, I always learned the students' names, and called on people throughout the lesson. This is tougher online.
Offered for what it is worth, maybe very little, by a retiree who does not have the problem now. I don't envy you this challenge. But I would suggest trying to avoid laying traps to catch the students, and would focus more on helping them navigate the new difficulties. Good luck!
