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First of all, welcome to the forum.
We need some voltages and signals from power on sequence.
Magsafe light should switch faster from green to amber (in seconds).
Post exact voltage for PPBUS_G3H and 3V3_S5/SUS, if appear.
You can touch PM_PWRBTN_L ball under U4900; use fine tip multimeter probes.
If you catch drop pulse there, when triggering SMC_ONOFF_L, the the problem is on U2300 side.