I agree with some of your points and disagree with others.
For LDAC: Yup, it is too efficient now. Even with upgrades the shots do miss, and are shorter range, but they don't miss that much. Nerfing LDAC to .8 or .75 would work imo. That said, I still get better performance with the 'elite' guns than this, as their other qualities make them better (and also more expensive).
For Ballistic Rangefinder: It is expensive and there are few ships which mount it to good effect - and even on those ships it is not mandatory - so I don't think it is quite right to 'bake in' its range boost when discussing small ballistics overall. It is a good choice on a Dominator if either forgoing frontal PD or relegating it to the medium mounts, and maybe on the Legion for the same (but that entails losing missile firepower). Using it on an Onslaught is similarly questionable as there are only 6 small mounts and they have poor convergence. Are there any other good candidates for it? I could just be blanking, but large ballistics are rare and the conquest doesn't have small ballistic mounts.
So, I really disagree with 'railgun + light needler have 900 range'. They do on a small set of ships where the build is setting aside 15 (for cruisers) or 25 (for capitals) extra OP to give them that boost, and using an inverted mount scheme to put PD on other slots. Or downgrading medium to small slots, which lowers a ship's offensive potential but can be made to work.
For mid range, it's funny because I think both heavy needler and heavy mortar are good weapons! The needler is just an upsize of the light, so can be swapped there if the slots work (they sometimes do, they sometimes don't). It's high DPS/slot, high efficiency, alpha burst, and range matching with the heavy mortar are nice.
For the heavy mortar, if the accuracy can be overcome (which as you argue for the LDAC it can be) it has good stats. Not as crazy as the LDAC, but .82 FPD at 110 HE shot size (220 vs armor), 220 dps, all for 7 OP. I think heavy maulers with their 1000 range are good guns, but for comparison they have 1.0 FPD, 120 DPS, 200 HE shot size, for 12 OP. If a ship's plan is to fight in the 'mid-close' rangeband, mortars are much better than maulers.
For the HAC: I feel that the railgun/needler/HAC line were fairly balanced before because of the OP costs, range, etc, so I would argue for the LDAC to receive a nerf rather than this gun receive a buff. I use this weapon and think it is ok. It has significant DPS advantages over the HVD, and is significantly cheaper than the heavy needler.
I have uses/builds for all the current large ballistic guns, though a few notes:
I agree with you on Mk IX and Hellbore. They are cheap, they get their jobs done. I think the hellbore is somewhat overated in how people talk about it, because its slow projectile misses a lot. Great at busting big ships armor though.
Haephestus: Better than people give it credit for, but 480 is a lot of flux to dedicate to HE. I feel like this has the stats of 2 perfect medium mount HE guns (with 100 more range) stapled together, which is not always what I'm looking for in a large mount.
Gauss: Unrivalled range and shot size - smaller ships can't afford to get hit at all by this, their armor can't take it even though it is kinetic. Fantastic on ships built to use it. The main issue is that current endgame enemies charge into close range, and only 1 ship (conquest) can both mount this and also kite them. I'll use this on conquests all game, and on onslaughts, legions, and dominators for everything but ordo hunting (and it does work on onslaughts there, but is just not as good as other guns because the ordos charge).
Mjolnir: slightly too flux expensive, but otoh it allows for a kinetic ship to have 'decent' anti-armor while sacrificing less shield efficiency to do so than mounting an HE weapon. On a legion for example I will go Mjolnir+Mk IX as the only guns and it does well. I've experimented with this on Onslaught side mounts as well, and it does good in swatting flankers or letting me broadside, though it is a bit expensive and onslaughts have enough mounts that mixing in some HE is not prohibitive. Same for Conquests, it lets them not have HE.
Storm Needler: specialist anti-shield and great at it - .7 efficiency and 500 dps. Replacing it with smaller slots does not always work, depending on mount layout. Really shines as the center mount of an onslaught vs ordos.