Thanks! There are a few games I would like to grab at 60-70% off
If you know what you want, then
Is There Any Deal is a good site to check out. It can tell you a game's historical prices on Steam, and on other sites, so you can know how often they go on sale, and for how much. There are a number of reputable sites that sell Steam keys (though do check to make sure what type of key you're buying, for example Far Cry games tend to only be sold with Ubisoft Connect keys).
Fanatical is probably the best, but
Humble Store,
Greenman Gaming,
Gamer's Gate, and others are also trustworthy.
The main advantage of a third-party key seller is price. The biggest disadvantage is not being able to take advantage of Steam's return policy.
EDIT: Depending on the game, it may go on sale at least once a month at various storefronts (e.g.
Ori and the Blind Forest), or it may only go on sale a couple of times a year (okay, I can't actually find a good example, but I'm pretty sure they exist.)
Also, Steam has week-long sales starting every Monday (mostly games from small developers or older titles), weekend deals, mid-week deals, and daily deals, all of which can have discounts equal to the major event sales. There's always
something discounted on Steam.