Believe it or not. This stuff works. Not permanent of course. https://www.autozone.com/sealants-g...V63BHAR1JzxICEAQYASABEgLFFPD_BwE&gclsrc=aw.ds
As Rover range suggested, you should try the crc head gasket fix. If you're only losing a coolant reservoir worth of coolant every 5k miles you have a very small leak, the ideal type to be fixed with this product. This particular product uses nano particles reinforced with fibers so if it does fix the leak, the results should be permanent barring another overheating episode. If you're assuming you'll need a new engine anyways, why not try it? I've had so much success using products like at-205 to recondition seals, Valvoline restore and protect to unstick piston rings, BG induction solvents to clean valves, that I am now of the opinion that many of these products work, but they must be used appropriately with an understanding of what exactly the products do. One thing best to avoid though are oil additives, i.e. anything that changes the chemistry of your motor oil. You don't want to mess with that.
Randomly, was talking to my Uber driver about my coolant / potential head gasket issues on the way back to the shop to pick up my LR4 - and he recommended exactly this product. I guess it doesn't hurt to give it a try. Although read some reviews and some people are complaining it clogs up the system and leads to more issues - any insight on that? And how do you know if it's working - other than hopefully seeing longer durations before the coolant reservoir tank needs to be filled back up?