This one can be incredibly buggy, it didn’t unlock for me when I claimed my 2nd (or 6th) Milestone group rewards. The way I got it to unlock, was by unlocking the Stadium Development 2 and SBC Milestones back to back.
If you want to guarantee you can unlock 2 back to back, you can do this with the SBC Group. Work towards another milestone group, and once you’re done you can claim the reward for that, then complete the SBC one straight after. The trophy popped for me once I submitted the 100th SBC.
Use the Bronze Pack method to work towards the 100 SBCs with the Bronze (and Silver and Gold) Upgrade SBCs. If you check prices on players along the way, you shouldn’t lose much, or any coins, I pretty much broke even doing it. Then leave the 100th SBC unclaimed until you complete another milestone group, and submit it after you’ve claimed the reward.