I have no stakes in defending Steam, but—you realize that if someone were cracking passwords left and right for years then the web would be full of complaints like yours? Everyone would know that it's a thing that's happening. Eternal questions would be pondered to the sound of Guard notifications, lovers would gaze at stars with faint notificationing in the background, and musicians and poets would compose songs to that tune.
Frankly a keylogger on your laptop sounds more plausible.
My belief is that this is a canary in the coal mine. We know that password systems don't work in the long run. There's been lots of reasons to get people away from passwords for day-to-day things. Some canaries are going to die sooner than others, and I have some ideas why this particular canary of mine died early. There are other complaints out there about Steam specifically of hacked accounts where passwords were "guessed" and then email accessed. Additionally, I have a pretty good idea of why, for what I think to be very dumb reasons, my account is a particularly well known to be "high value" account (going back to the parent comment way above that depending on how you value it, my Steam account is worth more than the PC I connect to it with). Steam itself is also in the weird "entertainment" place where it has bank-like features, but not quite the same pressure to have bank-like security (because it's just "games" and "hats"). The article here itself points to things likely written prior to 2012 that are still in active use today in Steam's login path (whether or not you believe age/tech debt implies "broken" most banks have upgraded their login systems likely four or five times since).
(My account is one of the oldest accounts on the platform, predating Half-Life 2's launch, and originally accessed via dial up internet. It has several now rare collections of games and at least a couple now "impossible to buy" games. Most critically to it being "well known" to have such value, it has several of the most rare/valuable "TF2 hats", which I think is incredibly dumb and that the marketplace is a huge gambling mistake, and those were known at the time when all Steam inventories were public [oh, the spam and phishing attempts that generated back when that was public and easily accessible]. My limited regard for the marketplace and limited use of it would make it somewhat obvious if I had "sold them" in the time since inventories allowed going private.)
As for a keylogger, specifically, I would go insane if I ever had to type 50+ character passwords. The keys you will log are Ctrl and V. Sure that opens up questions to clipboard logging and/or Password Manager incursion, but as I mentioned above, I have enough reason to suggest the threat isn't that sophisticated (in part because it is just "games" and "hats"), and paranoid circumvention in place already (even beyond the ones mentioned specifically in the above comment).
Also, there are plenty of reasons it might not be happening as badly elsewhere as it is happening specifically on Steam. Microsoft (and Microsoft Research) has made it very clear in recent papers that distributed password spray (where the spray is spread out over large numbers of IP addresses/countries/etc) is the number 1 issue right now in passwords, and that detection and blocking are crucial. Steam has argued in the past that such things are impossible to do at their scale. (Microsoft would argue today that their scale in Office 365/Azure AD/Microsoft Accounts has easily now dwarfed Steam's scale.) There's enough evidence today (as I already mentioned) that Steam still doesn't have those detections/blocking in place (and are relying too much on Steam Guard/2FA to keep accounts safe). (Not to get too deep into the woods of Steam criticism, but the argument may not be that it is impossible at scale but that it is impossible to prioritize it within Valve's notorious management culture.)
You could attempt contacting steam and ask if they know how many attempts have been made from different IP addresses in total for login to your account. I feel like that's really the only way to verify what you're proposing. Steam likely has logs of all the IP addresses that attempt login to whatever account.
I'm skeptical of what you're proposing because it's not hard to design a system that freezes mass random IP login attempts to an account after 'x' low number of random attempts and then only allow the past successful IP addresses to continue with a successful login. As well, as do an email verification if the password is successful but being used from a new IP address.
I have sent tickets to Steam asking for such corroboration. I've never gotten beyond a "don't worry Steam Guard seems to be working as intended" and general Tier 1 copy-paste responses.
So I figured I'd go read what's ‘password spraying’ that you mention:
> Password spray is the opposite of brute-forcing. Adversaries acquire a list of accounts and attempt to sign into all of them using a small subset of the most popular, or most likely, passwords.
Frankly a keylogger on your laptop sounds more plausible.