Question Consistent Up/down Ping Spikes

You could ping your router IP to be sure but I suspect this is common wifi interference issue. It is often caused by neighbors around you using wifi. There signal damages your and your router need to retransmit which takes some time and causes delays.

You would have to do test again on ethernet to see if it is the ISP or internet but I bet it is your wifi.

You have few options. Used to be you could try to change radio channels or move from 2.4g to 5g. You can still try that but there are so many wifi radio sources that it is unlikely you will find any that is not heavily used.