I’ve had similar problems when importing from sketchup back when I used to use it, and these days when I use a ton of booleans and apply them all, I’ll typically see some problems like this.
The first step I do is to tab into edit mode, select mesh > clean up under the top menus and go through each operation. If these problem areas are relatively constant throughout the mesh, make sure you tap A to select all before running the operation, otherwise you can just select the problem areas and run the operations locally.
you’ll want to mess with the settings of each of these operations with the tool panel that pops up in the bottom left.
just from looking at this, it seems like most of the problems would be with the vertices and floating geometry.
beyond that, if you can’t run the clean up operations, the remesh modifier may help out, but I don’t have a ton of experience with that.
Finally, since you’re open to paid add-ons, I would suggest HardOps/BoxCutter. both are Boolean heavy add-ons meant for hard surface workflows, but could potentially give you more fine control over what you need to do while being non destructive. they’re both well documented and have plenty of tutorials available on YouTube. the only drawback here is that it’s definitely a more manual task.
if you’re still unsure or if these methods don’t work too well, feel free to upload the .blend and I’ll check it out this evening/tomorrow morning and see if I can come up with any other ideas or just do you a solid and clean it up for you.