Bingo.
Take Jumbo Cactpot. As far as earning 1M MGP for the achievement (or just a weekly source of MGP), the average player will earn 1M cumulative MGP from the Jumbo within 2.5 years or earlier if they use Gold VIP Tickets to increase payout. And given the odds of non-1st prize winnings, the average weekly winnings is 7500 MGP. Sure, many will be around 4,000 (turn in early and/or use a VIP ticket). But those 4th prizes and a lucky 3rd prize add up and make the average payout closer to 7500/week.
As for the time it takes to earn rewards, I've done most of what can be done at the Gold Saucer - achievements and prizes. I still have 2.5M MGP leftover after buying the 4M MGP mount. Honestly, there really wasn't much to spend MGP on in this expansion. The 4M MGP mount was big, but prior to that, it was mostly small things. Some TT cards for example.
MGP is something you can grind if you want to, but for many players, the better way to do it is work towards a short-term goal and then keep doing it even when you don't want something so you have the MGP when something comes along that you do want. Do those weekly challenge log items, mini-cactpot, jumbo-cactpot, etc. There's probably an easy 100k/week from doing very little. Add up some of the challenges in the challenge log, maybe do the TT tournament every other week (that gets a bonus just for doing 10 TT matches). Do 3 choco races and win 1. Do 1 Open Tournament. Do a few GATEs for the log plus get the bonus for completing them.
Fashion Report alone gives 10k for just showing up.
Then your average Jumbo for the week gives about 7500 (long-term) and your daily mini-cactpot yields about 30k/week or 36k/week if you use VIP tickets. And then there is the usually at least once per expansion Make It Rain campaign where nearly all winnings double.
Lots of ways to earn MGP without grinding for hours on just one aspect of it.