I think it’s a great change that mining productivity research affects mining-replacement recipes:
https://mods.factorio.com/mod/SimpleSeablock/discussion/696e39e3ef7f723ea0b2f1bc

I was looking at tungsten carbide and notice that while its tungsten ore ingredients gets this bonus as expected, the carbon ingredient does not if harvesting it directly from trees. But in unmodded Space Age, carbon on Vulcanus is made from coal and surfuric acid, and that recipe is still available in Simple Seablock. At base costs:

• 1 carbon takes 2 coal + 20 sulfuric acid
• 2 coal takes 2 carbon + 20 lava + 20 sulfuric acid
• 40 sulfuric acid takes 0.8 carbon + 0.08 calcite
• 0.08 calcite takes 4 lava

So at first this process consumes more carbon than it makes. But these recipes (except carbon) get a prod bonus from mining prod research. So after research level 16 (costs a very attainable 14k science packs) (or some fewer levels with prod modules) this combination of recipes can export coal or carbon by consuming nothing but lava, removing the need for tree farming on Vulcanus.

Now, a complicated process that produces resources from nothing but whatever comes out of an offshore pump is very on-brand for (Simple) Seablock, so I’m not advocating for any change here. But this process felt unintended and perhaps worth discussing.

Yes, I am aware of the coal-carbon loop. It's also possible even without mining productivity if you've got high enough quality productivity modules in a cryogenic plant. The only real way to avoid it would be to significantly reduce the coal from lava recipe (and even then it would probably be doable with very high mining prod), but I've play tested with it at lower output, or even earlier builds didn't have the recipe at all and you needed to use the regular coal synthesis recipe on Vulcanus. That setup was just miserable, trying to get enough coal to feed into liquefaction was not fun.

For most players it's going to still make sense to start with carbon trees on Vulcanus since you don't have cryo plants the first time you visit, and getting productivity high enough that early in the game is tough even if it's not impossible.

Once you've moved into the later game and do have that level of productivity available, I'm generally fine with it if you want to build this loop. I'm not even convinced it's worth the effort, but if someone does find benefit with it, go nuts. As you said it still takes lava, which can't be barreled, and is therefore limited to Vulcanus only and there's no crazy loop you can do on a space platform or anything like that.

I am in mid game and have noticed that loop as well but it isnt worth it in my opinion. I use trees for carbon and ship in wood from nauvis for my vulc coal needs. Unless the positive loop gets massive I doubt I would ever use it especially with wood being able to turn into coal at a 1->10 ratio. Being able to transport the equivalent of more than 10 belts of coal via 1 belt at the cost of placing some furnaces at the start of any production loop that needs it is a trade I will make any day.

I am playing with k2so so I do have greenhouses to make wood on nauvis more efficiently but even without that I would still ship over wood for how compact of a fuel source it is in this mod.