You can use the GSX-RR racing bike for 4 of the weekly challenges, doubling up on the feats and freedrive distance, and then the main stage and difficulty 3 events.
I'm not absolutely 100% sure, but I THINK you also get credited with custom mode completion for summit events and main stage legacy events. On my alt accounts where I've only mainly only done the bare minimum each week for a few months, I have around 3500-4500 prestige tickets. It depends what you want, I'm happy to just be able to get the occasional really noteworthy item from the prestige shop, like the Black Panthaa R34. If you're a completionist who wants everything, then yes, you'll have to do a lot of specific grinding of custom mode to get everything from the prestige shop.
Having done the summit on 4 accounts, I've actually come to enjoy the events a lot more. On the Isle of Oahu event I got top 100 on an account that didn't even have a half nitro chemist set, just from getting better at it after doing it once per account on 4 accounts! It has been a bit like the helicopter summit in that I hated the bikes at first, but just spending more time using them made me better at all the events, which in turn made them more enjoyable.
Another tip is if you don't want to spend the credits on the Ninja H2, the KTM Red Bull Team isn't far off for the summit speed trap, and I somehow already had that on an alt account, I think it's perhaps included in a playlist bundle.