I saw from another thread that other mods in combination with Sodium like Phosphor and Lithium (what I use) are allowed, and this was confirmed by a moderator. In the "Mods" selection of a speedrun, the only mods that appear are Vanilla, Optifine, and Sodium (they just updated it to say Phosphor and Lithium yet). This would mean that BetterFPS is not allowed, and there are several restrictions in place for Optifine for 1.16+ as well, so I would recommend installing Fabric and using Sodium+Phosphor+Lithium.