Stanly blade, the standard shape, used free, and not part of a handle. I have done what you are doing, holding the blade very flat against the GRP and raising the blade angle just enough to lift off the sealant. The bead can also be cut at right angles with little force, until the lifted bead falls away. Very controllable action. Don’t be afraid to change to a new blade if there is any sign of dulling the edge.
If you are removing the window, then use a cheese wire. If just removing a bodge fix with silicone, then remove as much as possible and then coat with a silicone remover available from Screwfix or Toolstation.
If it’s actually silicone it’ll push out no problem. If it’s something like sika it might be more involved with other tooling.
The pros often use a multitool with scraper attachment for the job as it gets in there without damage.
Make a tool out of acrylic, a 45 degree blade. A one piece plastic chisel. It will not scratch gelcoat or aluminium, its softer. But you need to keep the blade sharp - which means sandpaper.