So what I found out that works quite well for redefining structs in files with Pluto is the following.
Have a Pluto cell with this
https://github.com/fonsp/Pluto.jl/issues/115#issuecomment-661722426
And then import your methods as
X = ingredients("../src/MyPkg.jl")
import .X.MyPkg = MyStruct
If you were now to change your source files, manually rerun the above cell and your method / struct definitions will be reevaluated.
It’s not as reactive as Revise.jl (meaning you to manually run the cell / import the methods again) but it gets the job done.