Though I like the solution provided before me, there is a faster way, and it is what we have been doing for many Madden's now.
-Go to download community files and jump to the rosters tab
-Sort it by DL's
-There is currently one titled 99Cowboys (created by the user "Tyko" not me), this has every player on the roster 99 all stats.
-Download that roster, and then be sure to go and load it as your active roster
-To load the roster, go to the "Customize" tab on the main menu. Select "Share & Manage Files", then select "Load & Delete Files", jump to the roster tab, and Press
on the created roster
-Select a new cloud franchise
-Use a custom roster, not the active NFL roster, and select the Cowboys. You should notice some players you might not have heard of will be the 3 listed, all at 99 overall, that's what you want to see here.
-Go to create coach, I chose to make a motivator, though that really isn't going to matter
-Go to settings, and turn it onto All Madden, salary cap off, injury and pre-existing injury both -off, and then make sure that under "User Team Help", just move that over to "Simple". That should set all of the sliders to Auto, which is what we want them to be.
-Start at the regular season, and then select SIM 10 YEARS.
-For me, the achievement popped at the end of 2021, I believe. Took about 5 minutes and I didn't have to touch anything after I elected to simulate.
Congrats! You just got all of your legacy achievements. (: Roughly 250 gamerscore, in 10 achievements.
EDIT: There have been multiple reports of people not getting any of the achievements, though they are winning all of their games, and sometimes, even getting legacy points. The best thing i can tell you (at this time), is to start over. Make sure you loaded the roster (Details are on the 5th mark on the guide) and then triple check the settings. It's an apparent bug with the game that I would anticipate will be fixed shortly, but who knows. That has worked for some, but not all. For those that have reached out to me, I hope we have found you a solution, for those of you who haven't got it yet, message me on XBL and I'll try and walk you through it personally. Best of luck to you all!!