Will barely start and knocking bad

A couple of things could have happened. Sometimes there is debris or old gasket sealer around the old valve cover gaskets that gets knocked loose when the valve cover is removed. Pieces of hardened gasket could also be just barely hanging on and are ready to fall off and down to the bottom of the engine where no one will know about it. Often when the valve cover gasket is leaking it is because it became hardened over time and breaks apart causing the leak. Most mechanics will conscientiously clean or vacuum out any pieces of old gasket they see, but if a few chunks fall to the bottom of the oil pan that they don't know about, there is a screen on the oil pickup tube to prevent those chunks from going into the pump. If a chunk does make it through the screen it will end up trapped in the oil filter. If a lot of chunks plug that screen, you would have noticed a problem while driving, not when restarting the engine. The symptoms would have been low oil pressure gauge or low oil pressure warning light, a gradual loss of power, and usually a loud clicking noise, then a knocking noise. By the time the knocking occurs, the damage has been done to the lower engine bearings. There is almost always some indication leading up to that but we often don't notice and just keep on driving.

Another potential related problem has to do with an oil change. It is customary to change the oil after this type of service to get out any of that debris that might have fallen down. A problem can develop due to the brand of oil filter used. Cheap vs. Expensive is not the issue. It has to do with whether there's a check valve in the filter to prevent the oil from draining out of the passages while the engine is stopped. Some engines require that check valve in the filter. Some engines have the valve built into the oil pump and another one isn't needed in the filter. In some cases a previous oil filter without a check valve might fit on a different engine and the manufacturer will brand it for that application, but if it doesn't have that valve, some knocking can occur during the first few seconds right after the engine is started. Normally that is a little irritating but not harmful.

The fact that your engine made noises immediately after restarting it suggests something happened while you were driving it previously and the oil pressure isolated the moving parts from each other, (like it's supposed to do), while you were driving. Once stopped, it takes a few seconds for the pressure to come back up. That's the time most engine wear takes place but it's usually pretty negligible.

At 150,000 miles there is going to be enough engine wear in the bearings that the oil pressure will be lower than normal anyhow. It is somewhat common to use a slightly thicker, (higher viscosity), oil to overcome that wear. If your mechanic did perform an oil change, he might have used the oil called for. While that shouldn't be a problem, if bearing wear had already occurred in the past, the thinner oil is going to accelerate that wear.

To determine the course of action now, we first have to determine what happened. The shop that did the work should start with an oil pressure test to see exactly what the pressure is. That will determine whether the overall pressure is too low or just one oil passage is plugged. Next they may want to remove the oil pan and check the bearings. Bearings take time to be destroyed. That won't happen from running the engine for a few seconds. If all of the bearings show the same severe wear, that could have been caused at first from running low on oil previously, but it can take a while before it becomes evident. Bearing wear is exponential, meaning it takes a long time for a little wear to take place, then the wear increases at an ever-increasing rate until all of a sudden you hear the knocking noise. The way you described the course of events suggests something else happened besides normal wear, so the place to start is by calling the shop that did the work. It would be wise to have the vehicle towed there because if only the bearings are worn, they may be able to be replaced without a lot of other more involved repairs.

Don't overlook the possibility of something entirely unrelated too. A defective generator or battery can cause slow cranking, and the resulting low system voltage will confuse computers including the Engine Computer which is in control of idle speed. Those parts will fail at any time without warning and that would not be related to the valve cover service.