I hate to break it to you, but you lost everything like i did. My save also got corrupted like 2-3 months ago after updating the game, i managed to get ingame to do one tournament to get one of the new cars in the first week after the update, as soon as i finished the tournament and got through the rewards screen, my save was completly corrupted with no potential fix. The update was giving me issues initially, i managed to work my way into the game which ended up in total corruption of the game. Essentially the game crashed right after the rewards screen and that was the game over for me. I was playing on pc at the time, tried moving back to xbox one to solve the issue but the problem was pretty much the same on both platforms. I ended up deleting all forza games from my pc and xbox one because of this.
I also lost 1 year worth of progress, i had 220M+ credits, was close to 2 stars level rank (pointless but it indicates how much time i put into the game). I had 743/750 cars in the garage, about 10 cars in the auction selling or waiting to be deleted for being too long in the auction. I also had all the cars in the game except for 3 pre order cars, had 600+ cars that were modified by me or bought modified. Had close to 500 paintjobs handpicked by me with a few of them being made by me. The amount of tunes were probably in the 100s, i did like 10 tunes or so, and only kept specific tunes saved in my profile for resell purposes. Also was following a bunch of creators ingame for their decent paintjobs and tunes, which i assume is also gone which is a ingame thing.
I can also tell you that this issue exists since the very first forza titles in the xbox one generation, i googled for quite a while and have seen reports of people loosing their saves from FM5 and FH2. The big difference here is that, each new forza title increases the amount of data saved on your profile, in other words, the bigger the data, the most likely it is for the game file to get corrupted. The last 3 forza games are the ones with the highest amount of “corrupted save files” reports. Of course, there is also the aspect of increasing popularity of the games, the more people playing, the more likely it is to see these issues pop up. But even the most recent cases in FH4, that are mostly related to “save file corrupted after new update” clearly indicates that the larger the data, the more unstable it becomes for us.
I also chatted with support and they told me the only solution was to delete everything and start from zero, to which i replied with “No thanks, i’ll spend my time playing other games and in the meantime, i will never put money into forza games again.” I have bought all forza games, with the last 4 games i ended up spending around 100 bucks each time (sadly i didn’t even play FM6 that much yet i spent money on that game). I’ll be playing forza games on game pass from now on, for as long as the subscription remains a good deal to me. The support member i spoke with also stated they were already looking into this issue and had a workaround planned for the future or something, this is something i ignored and did not believe in, because this issue has existed since FM5 and FH2 and nothing has been done to fix it. And has the forza titles expand and get bigger, the issue becomes more common, this totally rings the type of thing that they ignored the issue untill it became somewhat popular in the franchise, which is the current case with FH4, FM7 and even FH3.
One last thing, when my save got corrupted, some other people got the same treatment as me, but there was one other guy who got his FH3 save file corrupted, he spoke with the support and for some reason he was able to get ingame credits and all the Horizon edition cars added to his profile as a compensation for the issue. I have no clue what this person did to achieve that but to me giving the horizon edition cars and a large lump of cash is not enough to compensate the lost of everything i had, so i didn’t even bother requesting that.