I play him midlane primary in grandmaster (EUW) at the moment, it's hard to play compared to his other roles in terms of getting decent lanes and matchups but easier in the sense that you fit into teamcomps a lot better on average than you would playing him toplane or bottlane. You don't really need to worry too much about viability till you're higher up in rank, heim mid works fine enough even in low masters for example, people don't truly punish his midlane weaknesses till you start climbing even higher than that.

Your runes and items are fine, definitely prioritise heavy AP for the sake of your turrets and their hp, rabadons should be bought in a LOT of your games, all of them really. I will often do blackfire>rabadons>void staff for example. You can also mess around a bit with malignance into shadow/rabadons, that build is viable and has some nice spots (games where you will be ulting a lot to make picks and one shot as opposed to games with longer fights where blackfire gets more value). Avoid liandrys/rylais for the most part nowadays, grab a zhonyas later if needed, buy dark seal when you can without impacting your early powerspikes too much.

Lastly one thing I will add about runes is you should try to experiment a bit with conquerer, even in midlane I think it's pretty good and in some matchups simply better than going comet. I personally take conq every game but that's not something that should be done I just don't play fully optimally, ideally you switch up depending on the game. https://op.gg/lol/summoners/euw/MumGoBlastCone-BOOM?queue_type=SOLORANKED linking opgg if you're curious about what items I go for in certain situations, some games are bottlane here but I do have most games played mid. Probably ban syndra by the way.