How to efficiently extract Individual Plannings given a list of Tasks and Owners in google sheets?

I'm not sure whether you can do it efficiently using only formulas, but you can use Google App Script to implement your function optimally in javascript.

You can initially collect the set of owners and the list of all days, then construct the resulting table by iterating over the tasks.

Here is an example implementation:

https://docs.google.com/spreadsheets/d/1zk01y8wwLvjJPwc3ov8f2NDUMIEcxPWMNhciFXvCaVw/edit?usp=sharing

And the javascript code:

class Task{
  /**
   * @param {[string, string, Date, Date][]} tasks
   */
  constructor(cells) {
    this.project = cells[0];
    this.owner = cells[1];
    this.start = cells[2];
    this.end = cells[3];
  }
}
/**
 * @param {[string, string, Date, Date][]} tasks
 */
function makeTimeSheet(task_cells) {
  task_cells = remove_empty(task_cells);
  const tasks = task_cells.map(cells => new Task(cells));
  console.log(`Running makeTimeSheet on the following ${tasks.length} tasks`);
  const start = new Date(Math.min(...tasks.map(t => t.start)));
  const end = new Date(Math.max(...tasks.map(t => t.end)));
  const header = ["", ...days_in_task({start, end})];
  const results = owners_dates(tasks);
  const table = final_table(results, start);
  table.unshift(header);
  console.log(`Returning a table with ${table.length} lines and ${table[0].length} columns`);
  return table;
}

/**
 * In an array of arrays, remove all the empty inner arrays.
 * This is required to work on infinite google sheet ranges
 */
function remove_empty(arr){
  return arr.filter(arr => arr.some(x => x));
}

/**
 * @param {{start:Date, end:Date}} task
 */
function* days_in_task(task) {
   const day = 24*60*60*1000;
   const duration = task.end - task.start;
   if (duration < 0 || duration > 365*day) throw new Error(`Invalid task: ${JSON.stringify(task)}`);
   let d = new Date(task.start);
   while(d <= task.end) {
     yield d;
     d = new Date(d.getTime()+day);
   }
}


// Given a map and a key,
// returns the value associates with the given key in the map if it exists
// otherwise insert a value using the given default_val function and returns it
function get_or_default(map, key, default_val){
      let value = map.get(key);
      if (value === undefined) {
         value = default_val();
         map.set(key, value);
      }
      return value;
}

// Returns a mapping from owner to a mapping from dates to projects
function owners_dates(tasks){
   const result = new Map();
   for(const task of tasks) {
      const owner = get_or_default(result, task.owner, () => new Map);
      for(const date of days_in_task(task)) {
         const projects = get_or_default(owner, date.getTime(), () => new Set);
         projects.add(task.project);
      }
   }
   return result;
}

function final_table(results, start) {
   const day = 24*60*60*1000;
   return Array.from(results).map(([owner, dates])=> {
     const line = [owner];
    for([date, projects] of dates) {
      line[1 + (date - start)/day] = [...projects].join(", ")
    }
    return line;
   });
}