As the title says. When was 5E "golden age"? At least for you.
For me round 2017-19 Xanathars to Eberron. YMMV of course. Don't care about the adventures but the splat and campaign books I likevghe best are from that timeframe iirc.
For me, the cutoff point of "All the books here play nice together" is right at the end of
Mythic Odysseys of Theros. Theros was a crunchy book with new, interesting mechanics that worked with the underlying design philosophy of 5E (the Piety system and Mythic monsters). Up to this point, there were some key rules the designers respected (non-Warlock spellcasters don't get spell save DC increasing items, not all ability scores are created equal - a Dex bonus is more valuable than an Int bonus -, abilities are keyed either off of 1/short rest or key score/long rest etc.) that kept the game balanced.
Tasha's Cauldron Everything brought a silent change in game design philosophy that, IMO, has led to powercreep. Don't get me wrong, I think Tasha's had some great inclusions. The Ranger fix was great, the Sorcerer subclasses introduced a buff that should have been retrofitted to all past Sorcerer subclasses in the form of spell lists, and the flavour in almost all of the subclasses were great. That said, most of the new content was clearly overtuned, and the pattern has continued since then (maybe with the exception of the 1D&D Druid). The Peace and Twilight clerics were busted (and I've personally seen how the Twilight cleric trivialises dungeon exploration in actual play), the X/Proficiency Bonus paradigm silently buffed multiclass characters (as if the busted multiclass combos needed more juice), Rogues got the ability to gain advantage every turn with Take Aim... To me, it felt like the game's underlying assumptions changed, and the new game design paradigm was more "whiteroom" and did not account for some very real variables old design considered (Tasha's DC increasing items are the big culprit here, just see Matt Mercer rue the day he gave Scanlan the Handcone of Clarity to see why that's a bad idea).
That said, Tasha's also removed racial ability scores, and while I still think Tasha's bandaid solution wasn't ideal, it was something that needed to be done. I'm happy that that debate is settled in the 5E community, and I'm happy with the Culture/Heritage combo Level Up came up with in the end (with the ability bonuses being relegated to backgrounds).
Since Tasha's, I just can't trust that anything Wizards releases will be properly balanced for my game. The Great Wyrm dragons in Fizban's are undertuned for their massive CR and mythic status. Spells like Silvery Barbs are overpowered for their spell level. OneD&D needlessly standardises things like tool costs and each background giving exactly one tool and one language proficiency. The game just hasn't gone in a direction where the books are actually that useful to me beyond serving as inspiration (and then requiring tons of homebrewing). Currently, Level Up seems to scratch my itch far better. I'm still interested in what OneD&D will bring as well as the Planescape book, but to me, the high point of 5E ended with the release of the Theros book.
