Thanks for the responses, guys. Unfortunately, circumstances have changed since writing this list, and I will no longer be attending the tournament in question - mainly due to new restrictions on events and gatherings.
I did, however, manage to get a match in with it before I made this decision, so I can share some details as to how it performed. My opponent used an Ultramarine list consisting of Girlyman, the new Indomitus stuff and some other Primaris units, including two units of Hellblasters. We played Overrun, and to cut a long story short, I won by 64-47.
What worked/went wellSecondaries: I chose Engage on All Fronts, Raise Banners and Assassinate (he had 6 characters for me to kill) - all of which were manageable for me to achieve. I ended up scoring 12 points from both Engage and Assassinate, and 10 points from Banners - all of which helped me to boost my lead throughout the game. Although I started the last round with just my jets, I was already a good 15 or so points ahead by then, so the choice of secondaries definitely matters.
Hellions: Yes, believe it or not, they were actually useful - well, one of the squads was: I made the mistake of trying to use the others to tie up a Hellblaster squad in turn 1. However, the other unit was able to raise at least two banners before being killed in combat with a squad of Bladeguard Veterans (which, btw, are nasty buggers when buffed). And, knowing that they would die to a stiff breeze anyway, giving them Hypex and making them Red Grief helped with their usefulness.
Cursed Blade Wyches: Before, when using a second cult, I would always pick Strife over Cursed Blade, as I felt that more attacks would be more valuable to me against tougher enemies. However, S5 Wyches can pack quite a punch (I did go with Grave Lotus for both squads in the end) and their immunity to Combat Attrition tests means that taking them in squads of 10 is still a viable option.
Jets: All 3 of my jets actually did quite well, with all but 1 of the Razorwings surviving till the end. I think what helped was, as I didn't have enough vehicles/monsters for my opponent to auto-pick Bring it Down, they weren't a priority - or if they were, the shooting wasn't as successful - which in turn allowed them to count towards Engage on All Fronts (I only failed to score it on the final round). And only once did I need to put one in reserve, purely because there wasn't room for it to fit around the opponent's forces (as in, there physically wasn't enough space) - but it was back a turn later.
What didn't work/go wellBeastmaster and Beasts: Suffice to say, this was a waste of 110pts. The plan was to use them to tie up units in combat. However, much like the Hellions above, they encountered the Bladeguard Veterans - and although they took the final wound off of one of them, they were immediately eradicated in return. And the Beastmaster didn't fare any better, as a nearby Judiciar heroically intervened and ripped her to shreds. They have since been removed from my list and replaced with a squad of Haywire Scourges for future games.
Void Mine: This was more bad luck on my part, as it ended up being ineffective against a full squad of Primaris Intercessors - not that it would have done any better against anything else, but it only did three mortal wounds out of a possible nine (one model had been killed the turn before).
Overall, though, I think the list works quite well - focusing on utilising their mobility instead of trying to outlast or outgun my opponent paid dividends in this match.
@fisheyes you make a few good points there. Personally, as I mentioned above, I feel 10-man Wych squads could still work with Cursed Blade, given that they can't lose more than one to morale, but maybe the Red Grief squad could do with being a 5-man.
T5 Reavers are surprisingly tanky, so it took my opponent a bit longer to deal with them - they still died, but not as quickly as I expected. And I mentioned above about how my Jets, not being a priority for my opponent, were able to survive and continue to be effective.
Personally, I'm not too worried about screening for Wych Cults. With Covens I can understand it because they have the durability to be in the open - the majority of Wych Cult stuff doesn't, especially if an opponent decides to focus fire and clear a space. Plus, if the opponent has no deep-striking, then I've spent points on weak units that I could better spend on something more useful (see above comment on Beastmaster/Beasts). I'd rather that the units I bring be useful in scoring points than just as a screen to deny a potential deep-strike area that my opponent may or may not use.
Regarding the PA stuff, I personally find it all a bit useless - Test of Skill is perhaps the only useful thing for Cults, but then I'd have to pick another trait that won't be as useful. I think I'll be sticking to Codex stuff until our new one comes out (also means I only need to carry one book, which is always good).