How to salvage data from a fried motherboard?

What do you mean by “professional cleaning and technical attempts”. Was this phone already looked at by an experienced micro-solderer?

The thing about water damage/data recovery projects is that not every phone/data is recoverable. Sometimes the damage is at the CPU level and there isn’t much anyone can do. If you haven’t already done so, you need to have your phone looked at by an experienced micro-solderer that specializes in data recovery projects. There’s a difference between someone who does mainly straightforward repairs, for example backlight or charge circuit, and someone who can troubleshoot a board that doesn’t have “signature” failures. It’s like a mechanic who is fantastic at doing brake jobs and one who can rebuild a transmission. There’s nothing wrong with only doing brake jobs…as a matter of fact, I would prefer that they redo my brakes rather than the transmission guy. But when the transmission needs to be rebuilt, you go straight for the specialist.

Look locally for a reputable shop that does micro-soldering repairs. Ideally, you want to find someone local but good workmanship/experience trumps locality. There are many shops worldwide that do this so you shouldn't have any problems finding them. Talk to them and ask how they do their work. Focus on troubleshooting, as opposed to shotgun replacement.

If you can't find one locally, then consider mail-in repair. There are several reputable micro-solderers patrolling this forum.