Welcome back Ironshark! Really nice to see you back on here; I remember devouring all your tactical and battle report stuff from a decade ago to help me think more saliently about playing Orks with some competence. It was enormously useful material back then, so it's great to see you're around and getting involved.
I've been playing ITC events and NOVA-style missions almost exclusively since 8th dropped, just dove straight back into "Competitive 40k"TM with the Orks when the rules dropped. In ITC, Kill Points are not Units Destroyed but Power Level Destroyed, so the Guns don't become a massive problem in those scenarios, and once you don't have to worry about that potential pitfall, I must say, Mek Guns are some of the best units Orks have got!
One of their best tricks is just the presence of the Grot Krew, as alluded to above. They're easy to hide and hard to target, and it's quite correct that they don't all need to hang around the guns. These guys have won me multiple games at events just by hiding on Objectives, out of LOS and with other models of mine up-field to prevent even things like Mortars from shooting at them. I've played against Chaos Hellforged Falchions with Prescience and nearby Lords that just rip my army off the board, but come out on top because he couldn't find the little Gretchin in time to stop them from scoring me all sorts of points.
Just this weekend, I played against another Ork player in the final round of an RTT, where we were sitting 1st and 2nd with a double-digit points-gap between myself (I was 2pts behind my opponent) and the 3rd-place player (Orks are back!). My opponent did something very clever with his Grot Krew, which is that he deployed just one unit of 5 behind all 5 of his KMK, and wrapped the rest up and around the edges of his deployment zone-- effectively preventing my army from getting a charge on anything except Krew should I have got the jump on him. It worked perfectly, as I was unable to get around his Grot Krew Bubble Wrap, and then he in turn was able to get the charge on me.
That's a fantastic trick to use against armies like Genestealer Cult, or Alpha Legion Khorne Berserkers, units that really rely on just showing up and immediately catching you in assault before you can do any damage to them; if all they're assaulting are Extraneous Gretchin, they're in some very big trouble come your turn!
Now, as to the guns themselves, and beyond little tricks you can pull with their Krew, I think the clear best choices are Lobbas for Big Guns and KMK for Mek Guns. Lobbas, unfortunately, are still just too expensive for what they do in the present game; why on earth GW thinks they're worth so much more than 'Umie Mortars I don't think I'll ever understand. But, that being said, they still do some good damage, and they can do it pretty much wherever you want them to do it, which means they have great utility.
KMK are the number one biggest stand-outs of all the gun-types for me. They're the best weapon Orks have, point-for-point, for killing high-T, good-Sv enemy models, be they Razorbacks, Taurox, enemy Flyers, or even a Primarch.* KMK have the shots and the BS to get a good number of consistent hits on targets, and have the AP and D to consistently turn those hits into some wounds. They can occasionally really flatline, but, I dunno, I guess that's Orks? I did 17 wounds to my own guns in one game, which was amusing. That's probably an outlier, though!
I include at least 4 KMK in every list I make right now for events, and they really do a lot of different things for me. Certainly recommend giving them a whirl.
*At least unless you add Wyrdboy Smites into the mix... nothing is better than automatic Mortal Wounds! Wyrdboyz are dumb, Smite is just too good, et cetera, and so on.