Dropping the difficulty doesn't really help when the drop rate is so insanely low that it'll take you hours to get what you need. Meanwhile, you have a finite target for the club and its ultra easy to do. There really is no comparison, the club is far less frustrating to do, no too ways about it. Both are a grind, but one you at least have an idea on how long it'll take, while the other can take anywhere from 10 minutes to 10 hours depending on your luck.
Later random encounters are actually worth doing. Mainly just because they chuck super high level enemies (and tons of them) at you at once. Really makes the combat shine when your taking them down with a mix of counters and perfectly executed combos.
Buggy parts aren't actually all that bad to get. The parts you need are fairly easy to come by, and even the modding parts you need are not too hard to get, a bit of a faff to learn all the locations of the traders and get the sweet spot on what items you need to trade for the items, but I didn't have a bad time with that.
The random super rare item drops in random encounters though had a trick to them, as they only spawn in certain locations, and even then you have to wait for the sports cars to spawn, then hope you get the item you need. Still, I got lucky with the random encounters and got the items fairly quick, especially compared to the hours the arena took from me...
And don't even get me started on getting all the base game 3 star talismen. Those take even more grinding as you need a ton of money and certain items that can only be acquired by racing, fighting or the casino. So really, that knocks the club down to third grindiest part of the game for me, if you aim for 100%. Ah the memories! I'll do it all over again when I get the time.