90c. is not all that bad.
The throttle point is around 100c.
But, overclocking is going to generate more heat that you need to control.
I think your small aio is not going to do the job on two counts:
- The radiator is too small to be truly effective.
- AIO coolers do not last forever. In time, air intrudes and they need to be replaced.
What is the make/model of your case and what is the fan arrangement?
Any cooler and graphics card needs a good supply of fresh air to do the job.
At a minimum, two 120mm front intakes.
As a test, take the side cover off and direct a fan at the innards.
If this helps much, look first to case cooling or a better case,
On air coolers, noctua makes very good ones.
Here is a chart for which coolers are suitable for a 8700K
https://ncc.noctua.at/cpus/model/Intel-Core-i7-8700K-404
On the list, a NH-D15s is likely to be the best.