Try working on your skills in evaluating a position. This will improve your ability to find tactics and spot weaknesses which then leads to candidates. Something I also look for are preparatory moves, candidates that are relatively quiet in themselves but prepare a much stronger next move.
Candidate moves

It's possible to provide any candidate move with concrete reasoning, but it's often experience that plays a major role in the decision-making process.
As an example, what should White do in the following position?
There are no obvious tactical possibilities or pieces to improve. In short, White's key idea is e2-e4, which can be played right now or delayed for a move or two. It's not easy to explain why this pawn push is the best option; there are pros and cons that requires an elaborate, time-consuming thinking process to be weighed precisely. But professional players don't have to do that. They simply know from experience that e2-e4 is the idea in this pawn structure.
The experience can be gained by playing through piles of high-level games, studying openings/middlegames and trial and error. It saves a great amount of clock time for the critical moments, where you have to make decisions based on concrete calculation.

Any tips/resources on choosing candidate moves when there are no obvious tactics/pieces to improve on the board? Sometimes I find myself really struggling to find a strategy when things are equal and there are no obvious improving moves or tactics to be found.
You could try reading the first two pages of this thread:
https://www.chess.com/forum/view/general/gm-larry-evans-method-of-static-analysis
Any tips/resources on choosing candidate moves when there are no obvious tactics/pieces to improve on the board? Sometimes I find myself really struggling to find a strategy when things are equal and there are no obvious improving moves or tactics to be found.