Like Klojum listed above there are a lot of cheap Android boxes out there. You really just have to do your own research and then pull the trigger on what you think will be the best bet for your uses. However, in the interest of your original post to get some examples of what works, i'll let you know what i run at my house. I also have a wife that just wants it to work and i also have 2 kids (age 5 & 7) that can use my configuration.
First, i have a main "server" machine which is just a computer always running at my house which runs a Kodi version matching all my TV boxes with the library being stored in MySQL. This is so that i don't need all my low powered devices scanning my media files separately, and i also gain the ability to stop a show on one TV, then move and resume the same show on a different TV.
Secondly, I got lucky and bought 3 Nexbox A95x boxes with a Samsung Evo Select 32GB MicroSD card. I am currently running LibreElec 8.2.3.1 (kszaq build still available on the forums) directly on the MicroSD card. Then i configure the boxes to use a custom remote config so that i can get my harmony to work seamlessly (have the ability to use play, pause, stop keys, etc). Then, i configured it to use the MySQL database for the media library. The entire setup took me a little bit to get right, and I've simplified my steps here for the sake of keeping it short, but this is working flawlessly for me. I have been constantly getting x265 content whenever i can and transcoding things with handbrake when applicable as well. I love the quality and file size of x265 and wish that all releases were done in x265 now that i have these little beasts.
As for the reason i mentioned the kids is because i was able to configure a separate Kodi user on my system using a separate media library database specifically for the kids. It's not the most user friendly to manage what they can see on their profile, but that is for me to deal with on my server. The kids just know that the system starts up to their media immediately and they can watch what they want. Then, i have a button on the remotes which switch the user to the "Adult" profile with a passcode and everything. Very slick and everyone just loves it.
Now, my Nexbox A95x boxes look to be still available from Gearbest, but they still run the S905X chip, and it looks like the S912 is the newest one being developed against. However, like you I don't have any UHD capable TVs, but i do know that the S905X chip can handle some of that as needed. Also, one last comment about the systems: I have them all hardwired to ethernet. The wireless in those small boxes are not very consistent and trying to pull large media over wireless generally causes issues.
Last thing to note about these LibreElec builds, and i'm not sure if they fixed it in future builds yet. When you play x265 content with vertical resolution which is not a multiple of 16, it results in a very dark black horizontal bar below the picture. This doesn't cover up any picture or anything, and depending on how your TV is calibrated, you may not even notice it, but I have been told it is an issue with the x265 decoding driver or something when the vertical resolution is not divisible by 16. I re-calibrated my TVs and now i can barely see it anyway. Don't know if it is fixed yet, but the rest of my family didn't even notice it when I would anyway, so it's not a big deal.
Again, take all this with a grain of salt and do your own research and determine the best option for you. If you want any more details from me let me know (i'm not an expert though, i just know how i configured my setup).