I also think, given your limitations, re: costs and time for "simple" mods, it will be tricky to get to the bottom of everything and a remedy which fits into your ability to do the work, mostly yourself and without incurring costs for constant hauling in and out. So, perhaps, just perhaps, a course of action maybe this:
View attachment 202731
Remove that 'blip' aft, and then
extend the natural lines of the hull aft, like shown. Try not to make it too fine, a nice lazy curve that
would gain back the buoyancy 'lost' by removing the blip.
Then
add a bilge keel, shown in blue, which is made from 2 plates space around 100mm apart at their base, where it is welded to the hull, with some simple internal ribs, which allows for plug welding to "
join them" together, so it has stiffness/strength. Make it a very simple symmetrical shape, nothing fancy needed.
View attachment 202733
Lining the plates by eye, by standing in front of the bow and cast your eye along where the
natural curve of the hull plating is going. Scribe that and use that as your rough guide to where to locate the bilge keel.
To be as far "outboard" as possible will help...so 2 locations noted, merely for ref.
Then on the
base of the bilge keel, have a thick bar that allows for bolting on ingots to add "solid" ballast.
View attachment 202734
This way, you can experiment with adding weight by 1, 2 or 3 ingots, or how ever many you think will help.
