Macro Lens - What to Buy?

Hello All 

I am thinking about investing in a macro lens and am totally confused by the variety available so could use some help and guidance. 

Immediate use of one: My wife is currently trying to digitise all our old slides and she is currently using my EOS90D with an 18-135mm canon lens at full zoom on a tripod and the slides on a light box so they are back light . It works ok but the pictures need a lot of cropping to get rid of the slide holder plastic/cardboard.

After this job I would probably use it for macro work in the garden on insects/flowers etc.

Do I go for a fixed lens or a zoom with macro. Some thoughts and suggestions would be very helpful. 

Thanks for feedback