King crab is - and will be at least for a while - my top performer IS assault, on level of KDKs except KDK-3. It lacks of structure/armor quirks to have a role in comp play, but for solo queue it's a brilliant and very fun to play mech.
As other king crab lovers, I could give some tips on it, but the important one you can extract from your own words. You say, it 1) has a huge ct 2) is very slow. Solution? Use big XL. 360XL for 100-tonner is a decent speed for an assault. That said there are good builds also for std350 and std335 (try to not go lower than 335 in solo queue).
-000:
6ac2
Boated ac2 are simply op for quickplay after heat buff. get some loong range free shots at the beginning of the match, then follow the pack and commit to pushes in the second line. If opponent shields and retreats - switch to another one, use your suppressive ability to protect your mates.
Unique option for the variant - 2uac5 + 2ac5 + 2ac2 - that cannot be done on a mauler. A little hard to play though and a little hot after ac5s heat nerf.
-000B:
2gauss + 3erlarge
Too common to give any tips. One of few IS gauss platforms that remains strong. Erl can be switched to regular ones - cant say there is one right answer now, after gauss nerf.
Another option for the variant - 4ac2 + 3lpl with XL340. Very big pinpoint burst dps.
-0000:
2lbx10 + 2lpl + 4srm4
Basically you wanna get in brawl with 3-4 mechs on each side, where your raw firepower can decide the whole match. But if the fight starts slowly, as usual for solo queue, you have a good midrange trading capability to survive for later push.
Another option, if you fear XL, would be plain 2lbx10 + 4srm4 with std350 or std340 and more DHS. You can lead a push and snapshot your 54.4 aplha every 2 sec for quite a bit. Cooldown is perfectly synced and 2 sec are enough to roll the return fire with your torso quirks.
People often consider atlas as a best IS brawler. But atlas is first and foremost a tank (one of the few in the game), while this variant of king crab has superiority both in offensive quirks and hardpoints.
Also i wanna say that king crab has very low profile for 100-tonner which leads to its "coming from nowhere" perk - perfect for committing pushes.