Shijou Saikyou no Daimaou, Murabito A ni Tensei suru

Both are about a demon lord re-incarnating although one is because of loneliness and the other is because he wants a better world. 

Both shows are about an MC who gets reincarnated in a new body in a different world or a future timeline. They take a really good take on the reincarnation aspect since both MCs are stuck with what they used to live as in their previous life, creating some awkward moments in the current society they live in, as they don't really fit in. So they need to learn to get accustomed to their new environment. 

Both shows are about a character who wants to have average levels in their next life but turns out to be overpowered anyway. Both MCs are also awkward as they aren't really accustomed to their new life. 

-A strong magician male MC -At least 2 main girls loving the male MC -Some ecchi with the girls -One of the main girls is a succubus -After every negative event, the positivity comes -Magicians are magic academy students -Something cute happens often 

Both are wholesome, relaxed reincarnation shows. The general mood is positive, and some troubles happen just to have more positivity later. Both shows have OP MCs. They are those satisfying, "can-beat-everything" characters. So if you're also satisfied with such MCs, these two have them. Both shows have MCs with cuuuute 😍 companions. They are generally female, unless we meet some male ones in the Reborn Demon Lord. 300 Years Slime does not have romance, but it's a very relaxed slice of life with wholesomeness. Actions are not very intense, and you feel that everything will be fine in a few minutes, even in the most troublesome scenes. In comparison, Reborn Demon Lord is also a harem romance, and has a bit more intense action. But you can still anticipate the positivity after every trouble. I admit, these may be a bit like kids shows in that aspect, but it's only as much important as you care. These are not plot oriented shows, so plot doesn't need to be strong like a mystery show. If you expect a flawless plot, these are definitely NOT FOR YOU. You may think these plots are like strainers because of so many possible plotholes that may be there. I don't even know how much, since I just ignored almost all of them. In general, I think both shows are for those who don't care too much about a strong plot, and just want to enjoy a relaxed show; even if the plot is not very strong. read more 

Both animes are kind of about the main character quitting thier job and both the fight scenes and comedy is a masterpiece 

-Both stories involve an overpowered MC, -Both stories have the MC surrounded by girl companions -Both stories are consisting of pretty much no negativity, except a little to make the show interesting -Both stories are cute -Both stories are consensual and wholesome polygamy romance. Note that both stories are definitely not for those who don't like non-novel plots, or not very strong logics. 

Strongest person in the world purposefully reincarnates for a second, more satisfying life.  

Thae same school setting that involves guy with great skills who want's to go low profile and same guy with a 2 girl harem. 

similar core concept of rebirth as overpowered protagonist 

Both feature an op isekai MC on steroids and a harem somewhat.  

Two super magicians reincarnated in a world with a "low" magic level. Basically your usual overpowered MCs story. The only difference so far is that Kenja is an Isekai and Shijou isn't (the MC reincarnates within the same world). Also, both have this initial magical school delepment to introduce the characters, harem included, before jumping into the "fight demons adventure" usual in this stories. With that said, both are actually entertaining and "better" written than "Shikkakumon no Saikyou Kenja". So if you like one, you may enjoy the other two. Just give them a chance. 

The same vibes I'm getting from both anime. The 2 main heroine is particularly the same. The outfit very similar.