Hello, thanks for the detailed info.
The code reported in the spark report is about chat packets.
You probably have a lot of plugins/players sending large messages, heavy colored messages or messages with lot of hover data.
Tbh I doubt the TPS drop is caused only by this since chat is async.
What you can do is trying to disable the chat features of ItemsAdder in config.yml:
font_images:
chat:
enabled: false
effects:
chat:
enabled: false
Test it and make sure to generate a new spark report, also check TPS.