I've been trying to determine whether I think there really has been a definitive "peak" for 5E and I don't think there really has been.
Imo there have been excellent AND weak official offerings throughout the run from the very first year. For instance, if you believe there has been a recent decline in hardcover adventure books in recent years, you may be viewing stuff like Horde of the Dragon Queen, Princes of the Apocalypse, and Out of the Abyss through rose colored glasses.
If you want try to do it by year:
2014
Starter Set - excellent
PHB - excellent
Horde of the Dragon Queen - okay
Monster Manual - excellent
Rise of Tiamat - okay
Dungeon Master's Guide - okay
2015
Princes of the Apocalypse - kinda bad
Out of the Abyss - mixed bag, but overall I'd say bad adventure with some great ideas and set pieces
Sword Coast Adventurer's Guide - okay
2016
Curse of Strahd - excellent
Storm King's Thunder - very bad
Volo's Guide to Monsters - excellent
2017
Tales from the Yawning Portal - okay
Tomb of Annihilation - excellent
Xanathar's Guide to Everything - excellent
2018
Mordenkainen's Tomb of Foes - okay
Waterdeep: Dragon Heist - terrible
Waterdeep: Dungeon of the Mad Mage - good
Guildmaster's Guide to Ravnica - good
2019
Essentials Kit - good
Ghosts of Saltmarsh - good
Acquisitions Inc - good
Baldur's Gate: Descent into Avernus - kinda bad
Eberron: Rising from the Last War - good
2020
Explorer's Guide to Wildemount - good
Mythic Odysseys of Theros - good
Icewind Dale: Rime of the Frostmaiden - excellent
Tasha's Cauldron of Everything - very good
2021
Van Richten's Guide to Raveloft - good
Candlekeep Mysteries - okay to good
The Wild Beyond the Witchlight - good
Fizban's Treasury of Dragons - bad
Strixhaven: A Curriculum of Chaos - very bad
2022
Critical Role: Call of the Netherdeep - kinda bad
Mordenkainen Presents: Monsters of the Multiverse - okay to good
Journeys Through the Radiant Citadel - very good
Spelljammer: Adventures in Space - mixed bag, but overall I'd say more good than bad with clear flaws
Starter Set: Dragons of Stormwreck Isle - good
Dragonlance: Shadow of the Dragon Queen - okay
2023
Keys from the Golden Vault - very good
I'm hard pressed to discern a clear "peak" there.
2014 was great but with the core books all in there it has advantages. It has arguably the best Starter Set in the history of D&D pulling a lot of weight, and 2/3 core books are excellent. Strong case here.
2015 has to be viewed in retrospect as an inarguable down year which makes 2014 look even more like a peak.
2016 has imo the best 5E adventure in Curse of Strahd and also the really strong Volo's, but if you have only 3 books released in a year and one of them is the terrible Storm King's Thunder is that really a "peak" year?
2017 feels like the peak for me personally because it's the year that I returned to the game after a 20+ year absence. It also happens to have 2 excellent books and no bad ones. But Tomb of Annihilation, while excellent, isn't the "best" adventure book and Xanathar's, also excellent, isn't the "best" sourcebook either.
After 2017, the release schedule becomes increasingly aggressive by year. So yes, you start getting MORE misses, but not proportionately more really. Waterdeep: Dragon Heist in 2018 I really thought was shockingly bad, and it made me worried about the quality control situation at WotC in a way that I hadn't been previously in this era. So you could argue that it "peaked" before that release? But it's not like it was all downhill from that point at all.
2020 imo is an underrated year for 5E books. It helps that I personally love the admittedly divisive Icewind Dale: Rime of the Frostmaiden. Tasha's introduced some much-needed reform to races and classes, along with a couple of badly unbalanced subclasses and pretty bad puzzles. Folks who ignore setting books, or non-FR setting books, missed two really, really good ones in Wildemount and Theros.
There is a rough patch starting in mid-2021 (Fizban's) through early 2022 (Call of the Netherdeep) that I can believe turned off some folks and made them think the best of the edition was behind us. But honestly I think there has been some strong stuff since then, and I think Keys of the Golden Vault is the best adventure product in a few years.