How To Install Every Ranked osu! Map
Prep:
Firstly, you'll need to decide whether you're going to add them all to your current osu! installation or make a new one. If you're fine with filling up your current install with 82k maps, then go down to the next paragraph, otherwise, there are a couple of extra steps. Download the latest osu! installer from The osu! Website (Don't use any existing osu! installers on your computer, older installers can sometimes cause issues), put the installer in an empty folder named "osu!" where you want the new osu! to install to (Don't make this in the same directory as your current osu!) and then run it. Once it's finished installing and you've set up the new client, you're good to go. Foreseeable issues with having 2 osu! installations, thanks to Death:

Death wrote:

The only real drawback I can foresee with this is that it might mess up your file associations. Meaning if you click on a beatmap or skin to open it, it might open with the wrong copy of the game. In theory it should open with the last install you played on, but that still means you need to remember which one that is. This can be avoided by only importing things by dragging them into the osu! window.

Downloading:
Next, downloading the maps. In my experience, the best place to download ranked osu! maps in bulk is from This Forum Post. It's very reliable (and trusted, it's a pinned topic in Beatmap Projects) and seems to indeed have every ranked map... up until the start of 2021 unfortunately. Unless I'm really dumb, once you get to 2021 you'll either have to download them (potentially) painfully slowly from the Official Beatmap Packs on the osu! website, or manually download them one by one on the Beatmap Listing or in osu!direct (supporter only). I chose the latter because of how incredibly slow the downloads were for the official beatmap packs, but if the official beatmap packs aren't slow for you, definitely choose those instead. If downloading from the Official Beatmap Packs page, you'll want to keep going back until you find "Beatmap Pack #969", this is the first pack that contains 2021 maps, with a few from the end of 2020. After that, you'll want to download every pack titled "Beatmap pack #***" until you reach the most current one. If downloading from the Beatmap Listing, search "ranked=2021", select "osu!" and select "ranked". Unfortunately, you will get rate limited (meaning you won't be able to use the site for about 20 minutes) every 201 maps you download (if you are going at a reasonable pace, if you go too fast it may be before that). Try to avoid getting rate limited, it's not the best. If this happens, like it says, take a break, and you'll be able to use the website again. If downloading from osu! direct, I'd wait until you finish installing every other map, then you can just make sure it's set to "osu!" and "ranked" and just spam click away. I really hope the bulk packs for 2021 are made soon, for everyone's sake lolol. (Also massive shout out to Elessey, the creator of that project, awesome work!)

Installing:
Next up is actually installing the maps, believe it or not, it's not completely straight forward. First, you're going to have to extract any compressed files you've downloaded (for example ZIP, 7z, or RAR files) into your osu! songs folder, which can be found in the folder where osu! was installed. You'll only want to do a year or twos worth of maps at a time to minimise the chance of crashes and such, but if you have a beefy computer feel free to try more. Once you've extracted a set of maps into the songs folder, either start up osu! and go into the song select menu, or if you're already there, simply press f5. Once the beatmaps have finished processing, I'd recommend deleting the non-o!std maps before doing another year. To do this, search mode!=o or mode={gamemode} in the search bar, right-click on one of them, and click "Delete all visible beatmaps". It's very important to make sure you only have non-o!std maps visible while doing this. Also, I'd suggest adding additional search terms such as lenght>60 for example, to get the number of visible maps close to 100. If you try deleting too many maps at once this way, you risk freezing and/or crashing your game. Then all you have to do is repeat these steps until you've got them all processed into osu!.

Finishing Up:
Once you've done all of that, you're basically finished! You'll want to wait for the Difficulty Calculation to finish, then search status!=r into the beatmap search bar. If any maps show up, just click on each difficulty with the Global Leaderboard selected and they will update their statuses to ranked. When I did it there were also a few "de-ranked" maps, as in they were ranked and then sent back to the graveyard, and some maps that were changed to loved, so you may want to delete those too. If you have osu!supporter, you can now open the osu!direct menu and download any maps you've missed from this process. If you don't have supporter and you believe you have some missing maps (like me) then um... goodluck! Since you've gone through all this trouble already, it'd be a good idea to back up your songs folder (or just your whole osu! folder) onto an external drive just in case. This may take anywhere from 3 - 10 hours depending on your computer and external hard drive's speeds, but in the long run, I'd say it's worth it.