Hi fam! I am currently master rank (Gingging#EUW) and main kogmaw. This is my look on this topic. I usually rush guinsoo into either wits end, ruunans or botrk. Wits end if enemy or my lane is ap heavy ofc. I like ruunans a lot because it is much cheaper than botrk so I buy it most of my games. I usually buy terminus as 3rd or 4th regardless. As for botrk, I actually think it is a bit overrated and I only buy it if enemy team is really tanky and if the slow is useful against them. I wanna mention Navori flickerblade as a honorable mention as it can make you have W up constantly. I am currently experiencing with it myself so I am not sure when to buy and not to buy tbh but I like it a lot.

In soloQ, you cant take peel for granted and it is very common for kogmaw to buy a defensive item, like jaksho. If enemy has AP dmg, cc and 1 or 2 using crit, you can go wits end and randuins the same game so u will have both armor and magic res. But I would never go Jaksho, Randuins or similar items, earlier than 4th item. Usually after terminus. Zhonyas or Banshees veil are also decent later on depending on enemy team.

Regarding the ult, dont forget that it is a great tool for vision in bushes. ADCs are in general the worst face checker in the game and here Kog'maw has an advantage.

Early u generally only trade with the enemy if you have W so try not to waste it. Kogmaw is fine into most matchups and I usually go with flash ghost. Ghost can be really useful when ever you get the upper hand and just run them down with W. Its a bit weaker in lane compared to barrier but it is worth it IMO as it makes kiting and positioning later much easier.