I would recommend basing your frontend "investment" around webpack. It's the common denominator that all the frontend frameworks use, and while it's fairly complex, it's complexity that only needs to be learnt once, and unlocks a lot of powerful functionality.
The frameworks themselves aren't that difficult to learn. Since Angular 2, they all work in a pretty similar way...
If by similar you mean they all have some component paradigm...I guess. But the cost of learning Vue, React, and Angular to a high level of proficiency is significant.
The frameworks themselves aren't that difficult to learn. Since Angular 2, they all work in a pretty similar way...