D&D 5E Melee Gish Tempest Cleric build help. Thanks in advance.

I'm making a Tempest Cleric of Thor because I love the flavour. My DM has informed me that the upcamping campaign will be hard and we should optimise to the best of our abilities. I'm kind of stuck to be honest.

Ideas I'm considering are Variant Human Magic Initiate to get booming blade and find familiar. Then get war caster, resilient CON, 20 WIS and 18 STR in some order.

I've also considered going dex since it's a better stat to have and Clerics don't benefit from GWM and Shield Master. Another idea I had was to use a High elf lineage Half Elf to get Booming and then take magic initiate to get Shillelagh.

I'm rather overwhelmed and would appriciate some insight from the Cleric aficianados.

So firstly I am going to break down what I am hearing from you; You like the idea of a Thor worshiping thunder smashing melee gish style character. You want to focus on Tankiness and Spellcasting while also doing well in melee. Your race is flexible. You want booming blade.

Going off of that it really doesn't sound like tempest cleric is the right fit for you :/
Tempest Cleric lends itself mostly to being a blaster, using almost entirely spells to do your damage. It can definitely be tanky but doesn't really give you specific features to help you there. It gives you basically nothing in the way of martial prowess, no fighting style, extra attack etc. the only feature that supports even carrying a weapon is Divine Strike and in my experience that feature is a joke. 1d8 per turn on your turn is just so minimal and you have to wait sooo long to get it. It stipulating on your turn, plus the fact that you won't get extra attack means that many rounds you won't get any benefit from the feature at all because you missed on your one chance.

For what it sounds like you want to do my recommendation would be to ask your DM if you can use the most recent UA for favored soul sorceror. Link And throw in 2 levels of Paladin to give you armor/weapon proficiencies and Thunderous smite, plus fighting style.
This hits a lot more of what you're looking for. You have that cool rp link to Thor and thematically are very similar, doing his will in this plane. You get Booming blade and Thunderclap without having to jump through significant hoops. You have actual features that bolster your offense and defensive abilities martially and spellcasting wise. You even get the super-cool nova turns of using Quicken spell to Booming Blade two different enemies, or the same guy twice, in one turn. Thunderous smite is super cool and on theme as well, you can preload as a bonus action and then booming blade to do (at level 5) 1d8+2d6 Thunder damage plus your weapon hit, and if they fail the save you knock em back and prone so if they want to get back into melee range they have to set off your booming blade.

There are some other options if your dm isn't agreeable to the UA stuff. Some of it in the past has been a little out of bounds so it's iffy. This version has been iterated on a bunch and it seems very balanced to me however so there's a good chance that he'll allow it. Let me know what he says and if it's a no-go I can dream up some other advice for you.

I presume that you want a character that is similar to the character Thor from the MCU. I personally like the idea of this character; it has a lot of roleplay potential while being effective in combat. However, I think you would have more luck as a Paladin. Paladin gives the divine flavour of Thor and puts the character into more of a frontline role which seems to be what you want for the character. It's certainly possible to reflavour Divine Smite to have lightning or thunder damage, as befitting the character. Aura of Protection makes you and any ally within 10ft of you nearly OP in terms of saving throws and the combination of Extra Attack and Divine Smite makes this character an absolute beast in melee combat, though you must be careful not to burn through your spell slots too quickly.

If you want to wield Mjolnir, multiclass as a Pact of the Blade Warlock with your bonded weapon being a Warhammer or a Maul. This gives you the ability to recall your bonded weapon as a bonus action, you can ask the DM if this can be flavoured as the hammer flying back to your hand like Mjolnir does in the movies. This gives access to Eldritch Blast (reflavoured as the bolts of lightning called down by Mjolnir) and Booming Blade in one fell swoop, though Booming Blade is less attractive seen as Paladin gives you Extra Attack. In terms of game mechanics, Warlock spell slots refresh on short rests and you can use Warlock spell slots to fuel Divine Smite, giving you a bit more staying power throughout the adventuring day, while Eldritch Blast provides a powerful ranged option that Paladins usually lack.