Alright, here's another suggestion.
As you know, Space Age adds molten iron, molten copper, carbon and carbon fiber. If LSA could "fuse" these items/fluids to their IR3 equivalent, I think it would be neat.

• Molten iron and molten copper already exist in IR3. They could be fused with little to no balance issue.
• IR3's coke is very close to serving the same role as Space Age's carbon: a pure form of carbon made from coal. Several of IR3 petrochemistry recipes support this (i.e. coke gasification is coke + steam and returns only H2 and CO2, so the carbon is coming from the coke). Fusing them shouldn't create a lot of balancing issues. Still, if you don't want to fuse them, then at least it would be good to have a recipe to swap between the two (maybe something involving sulfuric acid).
• This is a bit of a stretch, but IR3's graphene foils are pretty similar to Space Age's carbon fiber in idea. Still, their concept are very different (one is made from plastic in a laser assembler with helium/nitrogen closed loops, the other is made only on Gleba) and fusing them would definitely affect balance.

Your patchset, your rules. I'm only suggesting this because it's what I'm planning to do in my own Space Age x IR3 addon (eventually).
(Apologies if this new thread is a big spammy, but I figured it's better to keep these suggestions separated)

Molten iron/copper I might think about. The trouble is, which ones do I prefer, IR3 or Space Age's ones? Different add-on authors will have different preferences, which is why I left these sorts of recipe changes out of LSA.

(A reminder that LSA is not supposed to be sensible or even playable, it just puts the tech tree into a somewhat prepared state for other modders to work from.)

The only "fuse" that I've done is the Rocket turret (preferring IR3's) and I'm considering reverting it.

After thinking about it, I considered migrating the IR3 fluids to the Space Age ones for the tiny chance that it would cause planet mods to work, then I realised it would never happen.

So instead, I've hidden SA's molten iron & copper, and all of the foundry recipes are changed to use IR3's version, except for "Casting steel" which takes Molten steel.

("Casting low density structure" doesn't get special treatment because it's a non-sensible recipe that is guaranteed to be altered/deleted.)

The "Carbon" items I'm going to leave alone, that's up to you.