Thats basically how I got mine quickly.  I farmed Reeking Douglas for legendary and rare cores and every 25 I opened I would go and release all the commons I got (by this time I had all the field skill farm whore commons I needed just through the standard common cores and gameplay to the end) and then would use the boosters I got from that.

Also, not sure if you noticed, but you can get luck % boosts from common blades.  I think I had Rex's luck up to almost 800 when I finally popped my last rare.

For all its excellent qualities, I think this might be one of the most frustrating games I've ever played.

There are just so many little things that annoy the shit out of me; getting halfway through a fight only to have a mob of foes join in and fuck you up being chief among them. Tedious grinding aside, I've spent the better part of an hour abusing the game's detection and disengagement rules to cheat my way through areas because they're set up in such a way that trying to fight way through normally is pretty much impossible because every goddamn fight turns into a clusterfuck cos random enemies keep joining in until I've fighting about 20 and just get crushed by sheer weight of numbers. It's just maddening.

Have you tried using the lure button? I use it and run far away from a group. That usually does the trick. I also concentrate my attacks to kill weaker guys first. I usually never have a problem with mobs using those strategies. 

You are past chapter 7 right?  You can eviscerate large groups with a chain if that's the case.  If you can quickly get max affinity with Pyra/Mythra and full party guage, just get 2 elemental orbs, maybe 3 if the enemies are tough and whip out the full power mode and chain.  Level 2 and 3 specials are AOE, you will kill nearly everything in the near vicinity.

Yeah sometimes I am able to lure them out one by one, but even when I manage to lure them away to an empty area, somehow a bunch of other fuckers always seem to wander by and join in halfway through the fight. I ended up just running through everything to the next cutscene trigger with about 10 enemies in tow trying to engage me, which didn't feel like how the game was supposed to be played.

The problem is, I often can't get to max party gauge cos I keep having to use it to revive my guys when they get ganked halfway through a fight when new baddies randomly show up. It just got particularly on my nerves in Chapter 9, where as I say I ended up just running through pulling a daisy chain of angry baddies behind me cos any attempt to fight always turned into a ridiculous 10-on-1 clusterfuck. Ah well, I got passed it in the end I suppose, so at least I didn't get stuck. Really want to see how this ends.

Hmmm.  There's a lot of factors that go in to your success in these fights.  So I can't really tell you where the problem lies.   I will say some of the enemies in the area you are in summon reinforcements.  Best advice there is to go for a Seal Reinforcements blade combo.  That's a combo that ends in the dark element.  Another option is to shift focus to small badies when they show up and eliminate them quickly.  I also don't know where your affinity charts are at this point.  With Mythra, I have it at the point where every crit with an art nearly completely refills that art so I can fire off Light elements like crazy and use her to charge my special so it makes pulling off combos really easy and also makes building up party guage easy too.  

Oh and without spoiling anything major about the final boss, he is level 70 so keep that in mind.  

I would also check your Accessories, Aux Cores, and Core Chips.  Also, see if your characters have any upgrade points available for their affinity charts.  I strongly recommend going for the ones that have you start with arts fully charged.  Makes it easier to get off blade combos earlier.  

Also, are you using Nia as a blade or a driver?  If the former, definitely lead with Nia and giver her an Aux Core that reduces her cooldown time after you switch.  Also, when she's a blade I recommend going double tank with Tora and Morag.  For this strategy to work, you definitely want Poppi QTπ unlocked for Tora with her setup as an evasion tank.  It not only gives Tora another blade, she does great damage which helps him hold agro.  

All I can say is I didn't have too much issue, but early in chapter 9 I did step aside to do some side questing.  So I think I was probably higher level.

Thanks. 

Without spoiling it, does the final boss require a specific strategy or have any troublesome gimmicks I should know about going in?

No, but you should cain-attack it as soon as you can. Otherwise it could give you problems with reinforcements, which are what can make the fight a little harder. The final boss also has a one-shot sort of attack, but you can evade it if you use a chain attack, complete a Blade Combo or use a level IV Special. Also, if you use critical healing it becomes way easier. 

He has a hell of an opening attack in the second phase so have either Nia as your lead blade or have Healing Halo as an art that starts charged for Nia and Dromarch.  He also has a couple of AOE attacks that can be pesky.  The only other thing he does is summon reinforcements that are fairly strong but in my experience he doesn't summon them fast enough to cause me too many troubles but I had matched his level and had my blades pretty substantially built up.  I was rolling Nia, Mythra/Pyra, and Herald  (who is easy to build up to a crazy level btw)and had all pretty substantially built up.   My advice is that he has enough HP that you can pile elemental orbs on him before he rages fairly easily so mount up as many as you can, whip out ascended Pyra/Mythra and chain attack.  The damage output at 4 or more is very substantial, you can drop him when he is at or above half health.

Oh and fyi, if you are ready to do what I said at the end and he summons reinforcements, just launch the chain.  You'll kill the reinforcements easily even if he has enough health to hold on (though depending on the number of orbs, he should be nearly dead or dead even at 2/3 HP)

OK, not going to post my full thoughts here - other than to say I think it's a really, really good game and I like it a lot - but I do have to point out that DANG when Monolith Soft takes the gloves off and goes all out with their cutscenes, the presentation still kicks sooo much ass. The direction and cinematography of the cutscenes in this game are amazing when the game really gets serious.

Honestly, whoever does these needs to be brought in on more story driven projects across Nintendo. Though Nintendo in general has really shown they know how to do a cutscene lately. Mario Odyssey's story might be all fun and games but there's some fantastically animated sequences. And Breath of the Wild may have had fewer big cutscenes than it deserved but it could certainly put on a show. But Xenoblade Chronicles 2 is by far their best in this department. Honestly, I hope that as their projects get more and more backing, more and more of the cutscenes in each game get the premium treatment.