If you can get the bulk up like in the photo and have the 4” scraper as mentioned before, try and turn the blade over to the flat / dull side to get that residual stuff. Sometimes it works better than the sharp side that digs in.

Good news: even if it is asbestos, it’s most likely non-friable, and there is a less likely chance that it’ll become airborne versus cutting with a circular saw, or dry sanding.

https://www.fs.usda.gov/eng/toolbox/haz/haz07b.htm#:~:text=Category%20I%20non%2Dfriable%20ACMs,locked%20securely%20into%20the%20material.

We have used our regular mastic remover after the bulk is up and it will soften. And make it easier to remove, but not sure how much damage the chemical will cause in between the slats.

Good luck.