Fastest way to do this by far is with slight manipulation of the "new" browser.
When you complete a track that has no completions on it you are awarded 5200xp, and if you can find the right tracks in new you can complete a track in less than a minute and with load times (at least on the series x) I'm probably 15-20k xp every 5 minutes.
Before you start this achievement go and do the seven squared and build achievements and get them out of the way, and then you can just start grinding this one (most likely you will get the hot wheels and lego achievements passively like I did).
--> go to the horizon super 7 tab in the menu, go to challenge browser, and look at the "new tab"
Now the most frustrating part about this is finding an easy and quick track, lots of the tracks are just garbage and not playable, and lots of others are super try hard times that you likely would have to practice just to beat. When I was looking I focused on tracks with less than 2m completion in the description, and mainly destination or speed trap tracks (a lot of them were very easy or you could cut the track early and blow their time away). I also had very good luck with the ferrari 599x challenges as there were lots of quick easy ones on the interstate highway area.
Now when you complete a quick easy track, "like" the track and load up the new browser again. I think it takes a bit of time for the servers to sync up so a lot of the time that exact same track is going to be in the new tab again except it still won't have any plays on it (controller icon in the corner of the card) so you can just play that same track over and over again. I had a couple really fast tracks that I was able to play 4-5 times in a row very fast and was able to grind this achievement out in like an hour and a half.
Also a random little bug that's quite beneficial, when you cycle over to the "new" tab just go once more over to the "friends and followed" tab, and then go back to new and for some reason it will pull an extra 10 cards for you to look at as options, and there is an even better chance one of your easy tracks will show up again to play again.
Oh also a quick note if you cycle over to "new" too fast it will bug out and pull the wrong set of cards, so just cycle over to it slowly and give it time to load properly.
This isn't the most fun way to do it but there is also nothing I found fun about this dlc so this is how you just get it over with.