2016 C63 S AMG (M177) - Misfires won't go away, need advice

History:

• 50K miles: Replaced all 8 spark plugs (OEM as per maintenance requirements)
• ~65K miles (Nov 2025): Cylinder 7 misfire. Replaced all 4 coil packs + cylinder 7 plug. Made the mistake of not replacing all 8 plugs at that time.
• I am about 500 miles over the maintenance for the in-tank fuel pump that I haven't replaced yet, but I haven't gotten any codes about that at all yet but in any case adding here in case it's an issue.
• All other maintenance always done with OEM parts and serviced on time
• 67K miles (last week): Check engine light back on, took it to a shop

Shop findings:

• Borescope showed internal piston damage on cylinder 7, but said it's very minor and doesn't need to be addressed at this time
• Ground electrode destroyed on cylinder 7 plug
• Spark plugs on cylinders 1, 3, and 7 were LOOSE
• Shop replaced all 8 spark plugs
• Did NOT do compression test, ECU ground testing, or any TSB diagnostics

Result: Drove 30 miles after pickup - misfires returned, check engine light back on.

Current symptoms:

• Check engine light back on, don't know the new codes yet
• NEW: Auto start/stop now automatically disables itself on startup (never did this before)
• 67K miles total

With brand new coils (from a few months ago) AND brand new plugs (just installed), ignition hardware is eliminated. The loose plugs on cylinders 1, 3, 7 might have damaged the cylinder head threads. U-codes suggest ECU ground fault. Auto start/stop disabling suggests electrical issue.

Relevant Mercedes TSBs:

• LI54.21-P-062006: M177 misfires - requires ECU ground resistance testing (<0.5Ω), coil swap test, software updates
• LI07.61-P-069688: M177 misfires - intake/exhaust smoke testing, driver's side intake manifold crack inspection
• LI07.61-P-073583: M177 misfires - post-turbo exhaust clamp failures causing false misfire codes

Question: What should my shop test first? I'm thinking compression test all 8 cylinders + leak-down on 1, 3, 7 to check for thread damage, then ECU ground resistance per TSB LI54.21-P-062006 if compression is okay?

Any advice guys ? I am not a mechanic, all of my data here is from the shops that have done the work, my autel mk808s computer and my google skills - that's all I know about cars as far as fixing them. I am a computer engineer so help me out here please. It seems like 2 shops now failed to diagnose what my misfires were from, the first shop or the second shop, one replaced coils, another replaced plugs, yes I asked them for it but I also paid for diagnostics and specifically said I have misfire codes.