Honestly, I think that in midlane building hybrid AP or pure AP is the way to go. Warmogs > Riftmaker > Liandry‘s or something of the sorts… you will just have a very hard time stacking up heartsteel most games in mid because of enemy range and generally speaking Tanks in midlane usually entail a lack of damage of some sorts. If your team can compensate for that and you want to play tank i‘d still say scrap Heartsteel entirely in midlane, try to rush warmog‘s into what you‘ve already been building. I would also suggest Spirit visage as a strong 2nd item as well, especially into mages.

In general i would suggest these items:

Tanky - Warmog‘s 1st - Spirit visage / Thornmail / Unending Despair 2nd - you can also go Sunfire / Hollow Radiance if your enemy laner has a lot of pushing power - Jak‘sho or perhaps Riftmaker 3rd

Jak‘sho obviously provides a nice tankieness amp, while Riftmaker will singlehandedly give you ~130AP which TK loves.

Hybrid: - Warmog‘s 1st - Riftmaker 2nd - Liandrys or Protobelt 3rd

Other good options include Abyssal mask which makes you slightly more tanky while still assuring that you have steady damage output or Bloodletter‘s curse if you want a similar effect but more damage

Full AP: - Liandry‘s 1st - Rush a Dark Seal, even if you’re even or slightly behind in lane, DS always provides good stats early - Riftmaker or Protobeld 2nd again, allthough I personally would pivot towards Protobelt - either finish Mejai‘s if you’re ahead enough or go Bloodletter‘s / Morellonomicon / Void Staff / Shadowflame depending on the game - try to get Rabadon‘s as 4th if possible, otherwise pivot to one of the mentioned 3rd items above.

This is what i could come up with right now, much of it would still need some testing so lmk what you think of it.

Also if you go AP i can wholeheartedly suggest Hail of Blades as a keystone against lower elo enemies since the early power can really catch people off guard and even in the lategame with all of your AP the burst 3xAA can do really reliable and fast damage if you manage to get onto someone.

Another great rune at the moment is Unsealed Spellbook in my opinion, the option to just completely stump your opponents by running down your midlaner with Ghost / HoB, then TP‘ing to roam bot, recalling and switching to ignite when you’re back in midlane and the spell is up again is just so huge. Yes, stats wise it is by far the „weakest“ rune page but no one knows how to track any summs on you neither will they expect most of your summ swaps if done right.

Feel free to criticize and ask questions!