generally, IR3 recipes are set up in a way to encourage having seprate copper/bronze, iron, and steel/chrome bases with some things imported back and forth, but the compatability often requires iron gears in the steel base (iron gears rather than steel gears in the tanker ships and lifting bridges.
maybe the steel plates could be replaced with reinforced steel plates in the ships, and on the tanker ships, the oil tanks could be replaced with 240 steel plates and a few pipes (which are normally imported into the steel base for other recipes)
the engine units are fine because tanks and airships encourage either manual, train, or airship transport of engine units to the steel base.
Maybe 40ish steel rods in stead of rails on the lifting bridge would help.
The ports could use iron rivets in stead of steel rivets .
iron plates in the large steel pylon could be made of steel.
Oil rigs could be like geothermal heat exchangers in that they're made of iron-base materials despite being unlocked with steel science, and you would only need to turn the 150 steel plates in its recipe into 150 reinforced iron plates to make them fit there.

Looks like I had only done the early recipes, so I revisited them with your advice in mind. The new recipes are in 0.5.1.

With third-party mods, my goal is not to make the recipes too expensive, but just to give them an IR3 flavour. In particular, I understand that IR3 favours using intermediates instead of other buildings in recipes.

awesome! i really appreciate it!
when i played IR3 in 1.1 i was frustrated the ship recipes didnt fit the IR3 recipes. Neither mod was meant to fit together, so it's understandabe. But cargo ships things needed to be crafted manually, which didnt feel right

I found Industrial Cargo Ships, a 1.1 mod which changes the Cargo Ships recipes, and Deadlock even acknowledges it on the IR3 FAQ. The mod also changes the accepted fuel sources for the ships which I neglected to do.

I'm going to change the recipes based on these ones, unless you have any objections. In particular, Oil tankers will take 6 Small tanks in the recipe.

sure! i think i vaguely remember that mod not quite integrating the recipes into IR3 recipes, but i dont remember to clearly.
imo it's not ideal, but it does make sense. It's not like anyone's crafting more than a dozen or so tankers per playthrough anyway.