Do you feel that fennel was merely a slightly nicer way of writing neovim plugins, or did it in fact give you a significant boost in productivity/capabilities?

Difficult to say, as I did not measure anything. From the code I've written I get the "feeling" that it is more compact compared to plain lua, reducing (my) cognitive load.

Fennel transpiles to lua, it doesn't give more capabilities, I would say. The concept of productivity (and capability) is anyway confounded by so many factors, making the choice of programming language negligible (unless you pick one of the extremes, such as Brainfuck, of course).

Fennel can also be used to write games in the TIC-80 fantasy console (an open source project similar to PICO-8).