In introtech only, the larger LRM launchers have inbuilt heat reduction, so only the 10 and 15 would need to get just 1 ton lighter to match the 5s + equal heat dissipation.
20s are already in line with 5s, until you are out of introtech and using double sinks. And indirect fire is super OP as it is, if you know how to use it. I would not buff indirect at all, and think that SPAs for indirect should be outright banned if you want to have a balanced time, as indirect SPAs are hella broken because they remove player interaction. If your opponent isnt able to interact with the board state cause of improved IDF, why even play together. That said, lowering gunnery is just the best advice in general to reduce game time, but it applies to every unit, not just LRMs at skill 3.
ACs are too heavy for tonnage balanced games, not counting the advanced ammo that makes them amazing in their counter-role. But if Izzy is playing BV balancing, AC's weight and damage is not the problem. Unlike LRMs, they dont need to be internally consistent for things to make sense as each type of autocannon has its own range profile and job to do, unlike LRMs that are all identical.
Now, if you dont play with other unit types AND dont balance by BV AND play pre 3062, then yeah ACs are weaker as there is nothing to FLAK and precision isnt until 3062, so I would suggest allowing precision if you play mech only games and want to buff ACs instead of adding vehicles and flyers that slow the game down with more rules. Precision solves every problem and makes ACs top tier, so finding a different solution that changes record sheets when a solution already exists doesn't seem productive; just allow precision in your mech only games earlier then the lore states to account for not having combined arms for the ACs to shoot at.
The problem with LRMs is that the larger launchers are all disadvantages for losing some heat (and making less rolls) that you could handle with additional sinks,
if you even need to, with the saved mass. Making them a flat equivalent in mass-to-tube removes that problem
and provides incentive to bring larger launchers over stacks of smaller ones, thereby aligning the game rules with the demonstrated in universe design preference.
On the subject of Autocannon: you are correct in that their
performance statistics (range, heat, damage, ammunition) are fine as is. The problem is "design cost." Because they are
so ****** heavy, there are available superior options for everything an Autocannon can do in either missiles (mainly LRMs) or energy weapons
which can trivially be mounted in their place without making any sacrifices. that's the important part.
Especially in the Succession Wars where CASE is not a thing, the combination of very high mass (cutting into total payload out of proportion to their performance), very large size (making them, especially the 20, far too easily disabled by a lucky crit),
and the ammo volatility on top of that, and the Autocannon just can't keep up.
Dropping the AC 2 to 3 tons means it can no longer be replaced by an LRM 5 for long range (higher damage on average, basically the same effective range, indirect fire, loads of support ammos even in the Succession Wars), ammo for that, a Medium Laser for close combat, and 1-2 sinks for the additional heat burden (if you even need that.)
Dropping the AC 5 to 5 tons now makes replacing it with a Large Laser
actually cost you consideray, rather than being a no-brainer 90% of the time.
Dropping the AC 10 to 8 tons means that it is no longer mathematically identical to a PPC; if you yank an 8-ton AC 10 for a PPC you now actually have to make a sacrifice somewhere to keep the same heat curve.
Dropping the AC 20 to 12 tons keeps it a Big Heavy Gun, but means you can't simply replace it with a half dozen MLs (for brawling), two Larges (for midrange) or a Blazer (to maintain head deletion threat) essentially for free.
Similarly, cutting two crits off the 10 and 20 reduces how often the are disabled by a critical hit... and also, as a pet peeve, allows the King Crab to actually have its guns in the arms (by official lore on how split crits work, the King Crab 's AC 20s are in the torsos, not the arms.)
edit: and before anybody goes "but Gauss Rifles-", Gauss are 1 heat. AC 20s are *7*. If you can't find three tons by pulling a few sinks out, your design was probably hideously undersinked anyway.