I am curious what you find. Checking the fuel pressure than then potentially pulling the valve covers off and inspecting the valve springs sound like good next steps. Hopefully it is the easiest fix, which would be the fuel pump or pressure regulator won’t build enough pressure anymore.
If those things check out, the good AFR reading seems to point to something other than fuel???? The engine shuttering under light load with a high rpm backfire after those other things check out it is probably time look more on the spark control side.
With 1.5s and the lifters mentioned the valve springs probably are not getting all that much abuse, the 1.5s make the cam quite a bit less aggressive. Clattering lifters are hardest on the valve guides and then you typically expect extra oil use before it becomes bad enough to unseal a valve. If it wasn’t starting to use more oil my best guess it is not a mechanical issue, and that lifter noise variations you were heard could be coming more from something modulating how the engine was running.
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