Suggest to focus on the cause of the noticeable vibration at idle (presuming in neutral). That sort of problem is usually fairly easy to identify. Pretty likely either something that is rotating when the engine is running isn’t properly balanced, or the engine & transmission mounts aren’t doing their job. Suggest the shop do a standard mount test to double check. I know you say you’ve had them replaced, but the replacement could have been done incorrectly, and that will show up when they are stressed in the mount test.
If the idle vibration cause isn’t the mounts, next guess would be the engine is misfiring. Ask your shop to check for any diagnostic codes, esp those related to engine misfiring or incorrect air/fuel mixture.
If not mounts, and no obvious engine problems, next up, maybe there’s some sort of incompatibility with the harmonic balancer and flex-plate. The flex plate might have been replaced along with the transmission at 105K. Both of those parts are usually constructed purposely out of balance, but when combined and matched together, that make the entire engine in balance. But the amount they are both out of balance has to exactly match.
If nothing found still, probably going to have to remove the transmission for a look see.
Not many Sentra owners here saying what you are saying. You can see for yourself using the forum search feature, link upper right this page. Mostly if there’s a complaint about the Sentra, pretty likely about the automatic trans. Manual Nissan trans complaints very uncommon. I expect you are just having some bad luck.
If trans is discovered the problem, suggest to ask your shop if it is possible to replace the current trans w/a manual.