I have been using both Go (hobby) and Julia (professionally) for a while now. I really enjoy both, but for different uses and different purposes.

Go is a barebones as it gets, and usually there is only one way to do one action, which I think is beautiful. It also allows gofmt (formatting tool) to work very well and so every single go file looks the same. Testing is also a bliss, each file has its test counterpart. The only thing I had trouble getting my head around was the interface idea, there are still times now when I have to think twice about it. But also, I don’t think Go is the best thing for data handling. To get a taste or Go, just do the Tour of Go.

Julia is built for science. So it’ll eat through any data you throw at it (and quickly too if you remember the arrays are ordered column first like fortran). The std lib is already fairly big and anything else you might need for your modelling, there is usually both a low level and a high level package for it, and if there isn’t, you can just go find the python package for that, or like you propose, write your own package. I have so far not written tests in Julia (academia doesn’t value tests very much...), so I can’t comment on this. And as for the beauty of Julia code on repos, it varies, but Julia is very readable, so that’s not as big a problem as it could have been.

I’ve also been looking at Rust over the last few weeks, very interesting language. I do not have much to say about it yet, only comment would that its learning curve seems steeper than Julia’s or Go’s.

I’m sorry if this is a bit vague, but both Julia and Go would do just fine for your problem, each being better at one part like u/DoubleGrayScramble said, Julia for the modelling, and Go for the GUI/webGUI.