With my "upgrade" comment I basically referred to the fact that the FDL is about 5-6 times as expensive as the Vulture, but not "better" by this factor, as other Commanders in the past have expected, so this was leading to some disappointment (I went through this as well). The Vulture is the best combat ship you get for the money.
I also needed some time to get adjusted to the Fer-De-Lance flight model, though having had experience in a Viper Mk III helped. Compared to the Vulture, the FDL is more difficult to fly, but in my opinion more fun. Admittedly, after buying the FDL I was close to selling it again, but it grew on me over time.
My current
build.
The interdictor and fuel tank in the internal slots 1/2 can be replaced as needed. The military armour can be debated, so far I havn't really needed it and it's mostly extra weight. But perhaps some day I will be glad I have it, might be the difference between getting away and a rebuy.

With the military armour, I think further hull reinforcements are overkill.
The 4A sensors are in preparation for the upcoming patch. A chaff launcher might be an idea. Class 4 Fuel Scoop is a must for my sanity. Relatively big cargo space is nice to have for the occasional mission and for engineering.
Everything is engineered. While I do not fully engineer all my ships, the Fer-De-Lance as one of my two favorite ships got a lot of love and time investment.
Increased FSD range is a no-brainer. Dirty Drives 4 or 5 are great, put at least 3 on it for now (you can fly out to Maia with an Asp for the final tuning, same thruster size). Shields and Shield Boosters are resistance enhanced. Weapons are all grade 5 rapid-fire (might need to evaluate that again with the next patch), MC got corrosive effect (highly recommended), lasers have no experimental effects (was unsure what to take and couldn't be bothered going through the experimental effects grind).
I experimented with many weapon loadouts. A huge beam laser with thermal vent and 4x overcharged multi-cannons ripped everything apart but the overcharged MCs boiled my ship too fast. With different engineering, this weapon setup would probably still be very good.
A big plasma accelerator seems less effective, but is a lot of fun and offers some satisfying moments.
Unlike the Vulture, the Fer-De-Lance invites for experimenting with many different weapon loadouts. My current loadout is pretty standard, very effective but a bit boring. Thanks to module storage, trying out something new is much more convinient now.
If you can do it, fixed weapons are possible in the FDL. I just suck with fixed weapons other than the plasma accelerator, so I run all gimballed.