While testing I found both Sulfur blocks and Onyx ore lack a proper tool to break them. Also, I noticed in EMI that the Sulfur block does not properly display its drops, both the block and sulfur items won't show it (but Sulfur shows it's dropped from Onyx ore). Also, fortune does not work on the Sulfur item when breaking Sulfur blocks or Onyx ore, not sure if that was intentional
Now, I know renewability has not been added to these items yet, so I was going to suggest some ways to implement this, as well as a potential change to Onyx. I've taken a lot of inspiration from these materials real life counterparts but in a way I think suits the game and the mod pretty well.
Onyx is known for being a dark, very dark colored gemstone, and actually most often occurs in calcite of all things, so having it be in geode form makes a lot of sense, but I think that you could split Onyx away from Sulfur as the two don't quite have much to do with each other - well they kinda do. I think that you could get away with making the gemstone more like vanilla in order to have it be renewable. Keep the current Onyx textures, but rename them to "Dull Onyx", because most real world onyx does look grey or dull. That block would be what spawns inside the geode, along with a "budding" variant to grow Onyx crystals. But, interestingly, you can chemically form dull onyx into black onyx with sulfuric acid! So, you could throw in a recipe to combine Sulfur and Dull Onyx (since I figure adding acid is a little out of the way for this mod) and make Black Onyx, which could get a much darker block set to reflect the stone most people recognize. At present, the Onyx trim is only a little darker than Netherite, Black Onyx would be almost entirely black for the darkest trim possible!
As for sulfur renewability, of course real world sulfur is formed with water and that's lame cuz it fits the vibe of the Nether very well so I'm going to just pretend that doesn't exist. Since sulfur is meant to be a naturally occurring substance, it could still be found in veins similar in size to Coal Ore in the Overworld. Or, better yet, add a new feature to the Nether in the form of a volcano - a small one, that is. Sulfur tends to be associated with volcanoes or hot springs, as an example the Better End mod adds their own version of Sulfur that generates in giant hot springs (in the end though, obviously). If you go the volcano route though, you could add little deposits of Sulfur at the bottom, they would just be little mounds 10-15 blocks high with a one block hole dropping down 50 or so blocks into that deposit. However, to make it renewable, when a Blaze is killed by water damage, it should drop Sulfur rather than a Blaze Rod. This makes it an interesting goal for someone looking to get more of the stuff. At present though, Sulfur does not have a lot of uses, so adding a recipe to make Gunpowder and some kind of fertilizer would be my suggestion. Since mining in the Nether or moving a Blaze to the Overworld for an auto farm/bringing Splash Water Bottles to the Nether is kind of a hassle, it should be able to craft a significant amount of Gunpowder/fertilizer. Maybe one Sulfur and one Coal makes 4 Gunpowder, and crafting one Bonemeal with one Sulfur gives 3 Bonemeal back. Irl, sulfur actually tarnishes copper and silver. Now, I know your silver doesn't tarnish, but using Sulfur to oxidize Copper blocks without all the waiting could be an awesome feature! Also, it burns blue in real life!
While testing I found both Sulfur blocks and Onyx ore lack a proper tool to break them. Also, I noticed in EMI that the Sulfur block does not properly display its drops, both the block and sulfur items won't show it (but Sulfur shows it's dropped from Onyx ore). Also, fortune does not work on the Sulfur item when breaking Sulfur blocks or Onyx ore, not sure if that was intentional
Now, I know renewability has not been added to these items yet, so I was going to suggest some ways to implement this, as well as a potential change to Onyx. I've taken a lot of inspiration from these materials real life counterparts but in a way I think suits the game and the mod pretty well.
Onyx is known for being a dark, very dark colored gemstone, and actually most often occurs in calcite of all things, so having it be in geode form makes a lot of sense, but I think that you could split Onyx away from Sulfur as the two don't quite have much to do with each other - well they kinda do. I think that you could get away with making the gemstone more like vanilla in order to have it be renewable. Keep the current Onyx textures, but rename them to "Dull Onyx", because most real world onyx does look grey or dull. That block would be what spawns inside the geode, along with a "budding" variant to grow Onyx crystals. But, interestingly, you can chemically form dull onyx into black onyx with sulfuric acid! So, you could throw in a recipe to combine Sulfur and Dull Onyx (since I figure adding acid is a little out of the way for this mod) and make Black Onyx, which could get a much darker block set to reflect the stone most people recognize. At present, the Onyx trim is only a little darker than Netherite, Black Onyx would be almost entirely black for the darkest trim possible!
As for sulfur renewability, of course real world sulfur is formed with water and that's lame cuz it fits the vibe of the Nether very well so I'm going to just pretend that doesn't exist. Since sulfur is meant to be a naturally occurring substance, it could still be found in veins similar in size to Coal Ore in the Overworld. Or, better yet, add a new feature to the Nether in the form of a volcano - a small one, that is. Sulfur tends to be associated with volcanoes or hot springs, as an example the Better End mod adds their own version of Sulfur that generates in giant hot springs (in the end though, obviously). If you go the volcano route though, you could add little deposits of Sulfur at the bottom, they would just be little mounds 10-15 blocks high with a one block hole dropping down 50 or so blocks into that deposit. However, to make it renewable, when a Blaze is killed by water damage, it should drop Sulfur rather than a Blaze Rod. This makes it an interesting goal for someone looking to get more of the stuff. At present though, Sulfur does not have a lot of uses, so adding a recipe to make Gunpowder and some kind of fertilizer would be my suggestion. Since mining in the Nether or moving a Blaze to the Overworld for an auto farm/bringing Splash Water Bottles to the Nether is kind of a hassle, it should be able to craft a significant amount of Gunpowder/fertilizer. Maybe one Sulfur and one Coal makes 4 Gunpowder, and crafting one Bonemeal with one Sulfur gives 3 Bonemeal back. Irl, sulfur actually tarnishes copper and silver. Now, I know your silver doesn't tarnish, but using Sulfur to oxidize Copper blocks without all the waiting could be an awesome feature! Also, it burns blue in real life!