AAZ G3 Turbo Exhaust Valve - BJ94

I had the compression checked at Bosch as preparation for a potential engine overhaul. One cylinder showed approximately 35 bar, two cylinders showed 39 bar, and the fourth cylinder only showed 21 bar. During the pressure test, a leaky exhaust valve was detected. The AAZ engine has about 260,000 km on it, consumes slightly more fuel (approximately 6.6-7.2 liters depending on driving style), starts well, and runs very smoothly.
What is recommended: having the valves ground in, or should the valve seats be machined and new valves/valve guides/valve stem seals installed instead?
I would be very grateful for any tips.

Thankfully, the issue with the valves seems to be resolved. While searching for a nano additive for the engine oil, I was recommended the ERC System Cleaner for diesel engines. I ordered it and, a few weeks later, when there were about 5-7 liters of diesel left in the tank, I added one liter of the cleaner. I completely drained the tank until it ran out, then refilled it from a reserve canister (absolutely do not do this with a new common-rail or electronically controlled injection system!!! If you run the tank completely empty, the fuel injection system will be damaged and cost several thousand euros!). However, this is not a problem with the old distributor injection pump.
Subjectively, I didn't notice any change afterward, but the ASU (presumably an engine diagnostic test) passed with a reading of 0.5 for opacity. Today, I removed the injectors and measured the compression – and lo and behold: 34 - 34 - 32 - 34 !!
"It saved me a lot of work and money. The old compression pressure was probably solely due to carbon buildup: the excessively high pressure on the first two cylinders indicates carbon deposits on the pistons, while the drop in pressure on the fourth cylinder suggests carbon buildup on the exhaust valve. The latter was leaking during the pressure test, which caused air to escape through the closed exhaust valve."