There is no way I know of to reliably split tablets into numerous small equal pieces. It is even harder to get equal pieces if the tablets are not perfectly round.
If you let us know the specific medications we may be able to provide more specifics on those particular medications. As a vet who sees a lot of cats, it is rare for me to prescribe a tablet that needs to be split into eighths, even for a very small cat. If the dose is so small, there are usually better solutions.
A few potential solutions to the problem (though perhaps not to the question itself):
Check with your vet if the medication comes in any other form other than tablets. A number of the common medications we use for pets already have commercially available pet forms, often liquids.
Check with your vet if there are other similar drugs that could be used as an alternative. Obviously, this would not be a practical solution for every medication needed, but for instance if your cat needs an appetite stimulant, or a blood pressure medication, or a pain medication, there may be alternative medications that work as well but are in a different form or more practical tablet size.
Have the medication compounded professionally. There are many reputable compounding pharmacies that can make just about any medication you need into either a liquid or compound a small dose into a tablet. They can flavour the medication to make it more palatable. Yes, there is some cost to this, but it may be worth it for the ease of administering the medication to your cat and you not having to worry about splitting the tablets.
If you must persist with splitting pills into eighths, a pill cutter is the way to go. I do it exactly as you describe - cut into halves, then each of those into halves, and then again each of those into halves. There will be wastage, and discard any fragments that are much too small.