The final version of USSEP officially designed for Skyrim SE 1.5.97 is . Modern versions of USSEP (4.2.6 and newer) require the "Anniversary Edition" (version 1.6+) because they depend on core files—specifically the four free Creation Club mods (Fishing, Survival Mode, Rare Curios, and Saints & Seducers) added in the 1.6 update.
Keep your EXE at 1.5.97, your USSEP at 4.2.3, your SKSE at 2.0.19, and your mod list will outlive the heat death of Tamriel. Unofficial Skyrim Special Edition Patch 1.5.97