Big Sur: I'm not able to charge up to over 8%
The first thing I would do is verify the charge cable is capable of handling the wattage of your adapter. (Did you perhaps get an Air adapter or a non-apple cable?)

https://support.apple.com/en-us/HT201700

The second is check CPU usage. A runaway process could out run even a 61W adapter that’s using the correct charge cable.

https://support.apple.com/guide/activity-monitor/view-cpu-activity-actmntr43452/mac

Check in Activity Monitor - show all processes: the charging circuitry should be able to net charge past 80% regardless of your “optimize” settings. Only if you have massive energy usage, would this make sense - and on your Mac - it can run the battery down if it’s stuck in a loop - the charger is less powerful than that hardware running 100% on CPU and GPU in some cases.


The third would be to open a support ticket with Apple online to run remote battery diagnostics. They have the best tools to pic apart the health status and recommend any actions. A one time SMC reset is good but now that you’ve ruled that out, I would go straight to the authority to see if they can share with you anything specific about your battery health.

https://support.apple.com/en-us/HT211094

You can look in About This Mac in system information and as long as the status is healthy, you’ll have an official diagnostic log sent to Apple in case you find it degrading and want to seek warranty or out of warranty repairs.
Your case where it’s less than 50% and running but not charging should be very easy for Apple to diagnose remotely.