Regulars are going to roll their styles here but I see an easy fix and cause.
The caps become faulty. They can let coolant out and not back in. That can make the reservoir over flow very slowly.
The coolant filler pipe can get pretty yuck and not seal well where the cap fits. Then air gets in and that’s not great.
You constantly filling it up isn’t replacing leaked coolant. It’s replacing air in the system.
This can all be sorted by you with a $25 bleed bucket off eBay, a new genuine cap and some effort to clean the filler cap seat. Maybe some coolant and distilled water.
I think your car will be just fine after that and your worries disappear.
So, fill the reservoir with premix red coolant.
Then clean the seat/sealing surfaces of the filler neck.
Pinch off the hose to the reservoir by folding it and using a cable tie to hold it.
Fit the bleed bucket to the filler hole.
Poor in the red coolant and fill the bucket a bit.
Run the car. If someone can help with a bit of throttle every now and then, it’s handy.
It will take around 2 or 3 thermostat opening cycles before all the air is out.
(There can be litres of air trapped.)
Lots of fine bubbles and the odd big one come through. When the thermostat comes on, it’s going to bubble aggressively and the level drop so be ready to top up. You don’t want that to empty. You’ll need a few litres on hand.
Repeat the process.
Then cap the bucket.
Then remove it.
New genuine cap on.
Release the reservoir tube by cutting the cable tie.
Rinse the area down with fresh water.
It would be highly unlikely you have a major issue.
*If you still have the added water in there, I’d be dropping all the coolant and filling with fresh red coolant and then going through the above process. There’s a drain screw on the bottom of the radiator or you can release the lower hose.
Follow this guide
https://forums.justcommodores.com.au/threads/vz-ve-vf-alloytec-cooling-system-repairs.279976/
Other checks will be making sure your cooling fans are coming on. Inspecting around the water pump for dry pink crusty residue and also around where the coolant filler pipe goes into the engine at each end. It has a crap o-ring that lets go.
Few work shops know these cooling systems well enough now and generally do a poor fill and bleed regime.