What really helped me go to a new level of improv and reactive DM'ing for when the players choices trully go off book was setting up my live session with pristine cool scripts and api functions that improve our actions in game without distracting from the fun of RP and Improv or taking player agency away. Also the most important thing I did that expanded my improv and response time was learning how to best use the transmogrifier. Setup a session that has every single battlemap you have ever drawn up/Dynamic lightinged up and store it in a session called "Battlemaps" or store battlemaps by biomes, I just name my maps with a scheme and keep them all in one battlemap session, then transmog alphabetizes the maps and I scroll down to "forestspecialbridge" or "forestspecialcabin" or "Forest1", or "ForestCampsite". Also I do not use the default Monster manual, I spent 2 weeks making every creature have 7-15 tokenside default tokens and then store every single one of those creatures in a session I call my own bestiary and yoink creatures over whenever I need them during session. I also have a cool system in the game where if the players have a small chance of encountering a monster that is "unique" so I just flip the token and run a script that adjusts their attacks/saves/hitpoints on the fly. This really adds to the players experience as they don't trully see the guardrail when they drive off book with their choices. I have a massive set of tables and decks setup in my session. I have a table system that rolls biome based encounters for the 4 tiers of dnd similar to xanathars encounter tables, but expanded and tweaked a ton adding in monsters from many sources and fixing encounters I felt were dumb for their tiers. For the Magic Item tables, I started with the default tables then as I ran each adventure module and purchased the newer books, I sat down and systematically added them where I felt they belonged. For tables, you absolutely need the table export/import api. Sadly nobody can really share their own tables with this api as it is too close to breaking copyright rules. Same with encounter tables, but what I like about my encounter tables is: