Reddit reviews Yutaka Mini Garden Barrier

To protect your plants, I'd recommend this: https://item.rakuten.co.jp/ideaworld/a-010/?s-id=ph_pc_itemname or https://www.amazon.co.jp/gp/product/B00PVAQ2WO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

These are spray bottles with a motion sensor. I have used it successfully with canned air to train my cats to not sleep in the pans on the stove \^\^. The included gas (butane) is flammable but harmless when used outdoors. If the cats have a preferred path to come into your garden, placing another bottle there might be enough to deter them from coming. There is another device here that emits high pitched sounds that are unpleasant for cats when it detects motion. This doesn't require any consumables so it might be cheaper in the long run.

There is a couple of organizations that can help rehome cats. Just make sure you aren't calling the Japanese equivalent of animal control. (Assuming you are a decent person who is annoyed about the situation but doesn't actually want these cats dead.)

Have you talked to the breeder and "yasashii ojisan"? It looks like you still have lots of options before doing anything drastic and please keep in mind that the situation is not the cats' fault.