Hallo @Fermata!
Your asking about the issue of „monotony“ in your fugue.
Sometimes it is not an issue with the composition itself, but rather a question of the interpretation or recording. Since you are using four string instruments „played“ by your notation software, you „naturally“ encounter the problem that the entire piece sounds somewhat monotonous. Even real string instruments blend the sound very well, in my opinion, but are not as expressive for individual voices.
I can imagine, when played on a piano, the impression would be totally different.
As currently discussed in another thread
@PeterthePapercomPoser suggested to improve the recording by “fine-tuning” the articulation, dynamics, and tempo for each individual hand or even voice.
I also had a fugue which I considered to be „boring“ and I was nearly to get around and throw it away. But then I applied such tiny adjustments in the tempo, for example to make the first bar of the subject a little bit „swinging“, which dramatically changed the perception. Since I do so now with all of my piano works, I know that it is a lot of work to literally maintain two scores, one for the printout and one for the recording. But it's really worth the effort to achieve a sound that is much closer to a live performance.
Concerning the composition „as is“, I think there are no issues. With a short overview on the score I see that you already applied the necessary counterpuntual techniques (such as tonal answer, a recurring countersubject, inversion, augmentation, stretto).
As in 6/2 meter with mostly half and quarternotes (perhaps in an older, more Palestrinian style), it would be not appropriate to add passages (in episodes) or countersubjects with a faster rhythm.
Perhaps one could introduce a kind of diminution which doesn’t increase the rhythmic pulse by doubling the speed but is merely a rhythmic variant of the original subject preserving the quarternotes as the fastest ones, finally resulting in a 1.5x diminution (such as in Bach’s D# minor fugue from the WTC1 BWV 853).