Out of experience, I would say that both Signal and Telegram are the easiest to 'mass' migrate over towards. The reason being that the obstacle/ceiling is relatively low for most users. The heaviest weighing reason being that both services are free, and the user interface is not all that difference. From the two being, that Telegram is the only service out of the mentioned two that has a clear business model with a paid premium service. Which is a plus (pro), services with a clear business model are easier to understand as to how they justify their continued existence.
I returned to WhatsApp, simply because no one I know either is on the other platform, not interested in migrated, or never stuck around (due to multiple chat application use). I could keep trying and preaching, but if they don't stick around, I will only be messaging me, myself and I. Making the alternative services relatively useless to me.
I don't know if this is still the case with Element, however, you'd used to only be able to use that service if you hosted a server yourself or trusted a hosting instance (could be run by anyone). Which is a big ask, since there is no official regulation that could be properly applied, given to all that personalized data that goes through the conversations. It would be a bit like the fabled: "source: trust me bro!" argument.
I never used Session, but it does suffer the same problem as with Threema, and Element alike. Userbase. You would still need to have a second messaging application for the rest of your contacts if the convenience or contact methodology in everyday life is/could be a concern. Realistically, for the grand majority of the worlds' population, unless everyone's social circle also used that chat application, they would still need to go through that bothersome route of having multiple apps. Making moving over to WhatsApp quite meaningless, both for personal use as well as for business.
However, if payment is not an issue, then Threema is a great alternative. Can be used on nearly any platform** people are on, licensing can be bought through their website (private and business), Play store, Apple store and Huawei. But it does suffer from the same thing I mentioned earlier, userbase. A paid licence usually makes it an obstacle for users, even if it's just the sum of a cup of coffee at a café or a beer at the pub. May not be worth much, but for 50+ users, and they are not dirty of a paid licence but without hosting it yourself, then Threema is worth considering.
Always remember this, even with your own due diligence, "A (social) media platform is only worth as much as the sum of it's users....". Meaning without enough users in a given social circle, users will always return to the former application they came from.
** Windows, Apple, Linux, Play Store, Huawei, F-droid and Threema-website.