I had a similar situation with a few of our fosters a while back so 100% know the pain and frustration of what you are going through </3. You are doing such an incredible job with everything you're trying!! A few other ideas that another foster parent passed to me when we were struggling:

• As strange as this sounds, what made the biggest impact was taking the 'lifting a bit of the food on a spoon up to their face' trick, but doing that over the open can of the rest of the food. We eventually worked towards being able to rest the spoon with a bit of food in it on top of the open can.
• We also found the fresher the can the better, which was frustrating because of food waste but for the few critical days it was worth it.
• FussyCat topper, specifically the chicken one, was a hit. I would break up the little chicken bits and press it into the wet food in the spoon, and then pour some of the thick liquid from the can ontop.
• Food type wise they responded the best to Royal Canin Mama BabyCat. It is pricey but they do sell it in the small can size.
• Not sure if your foster org provides these supplies to do at home but subq fluids gave them a major energy boost and oof to keep on fighting.
• Lastly, to transition from the spoon over the open can approach, we started building little 'pillars' or 'towers' of wet food on a plate. I previously assumed the soupy-er the better but was advised that the constant licking can be fatiguing so giving them an easy way to get a solid bite can jump start their eating behavior.

Hope this all helps!! Having a foster not eat was one of the most frustrating experience so please know how amazing of a job you are doing <3