Thank you, this is the same instructions for the Samsung S20+. Just reconfigured my device and did the same on Gboard so hopefully this will resolve the SMS cursor issue. I never had this problem with my S9 or any previous Samsung mobile devices I owned.
Have you tried clearing the cache of the messages app and the keyboard? https://www.samsung.com/us/support/answer/ANS00077611
I turned off the swipe and also the cursor that other people have suggested. It seems to be working fine now, though I haven't given it a real test, though it has not jumped up to the beginning of this text nor when I did a short test of a text.
So far still good.