Yeah, I am not definitely not a tower defence player typically, so you will know better. These are just the suggestions I give to most action games if they don't have those elements. I think the particle effects could help you without getting too noisy, if you just keep the particles to a minimum. For the camera shake and hit stop, I totally see what you are saying. One way to do it is to reduce the fire rate but make each bullet be worth more, and then at least the camera shake might work. The hitstop maybe you do only for when an enemy dies or when player takes severe damage. If you take a look at the twin stick shooter roguelike enter the gungeon, you will notice that they have a subtle screenshake each time you fire a bullet, and the bullets fire pretty fast, and in that game it doesn't get repetitive, though they only shake when the bullet fires, not when it hits. It still may not work, but I feel its worth a shot to reduce fire rate and add screen shake for the bullet fire, and then minor hit stop in those two situational cases.
For the removing towers, let me correct myself, what I mean is that - in order to remove a tower you select it with LMB and then remove it with RMB, then if you want to place that tower elsewhere, you have to again select it from the shop, and then place it in the new spot. If there is a net cost associated with buying a tower and selling it (I apologize I didn't pay attention to that part as much), then I understand the game design and I agree with your decision on making the purchasing and placement of towers important. In that case, yeah the slow motion does make more sense as an option for you.
I would be happy to playtest a future build if you make one. Feel free to reach out to me, at any time, even now. I'm @rsaamar on the Lost Relic Games Discord Server. It's good to have fellow indie game devs to chat with on topics and for feedback.