Some of the best gear in ESO come from their paid DLCs and Chapters. Technically locked behind a paywall, but you’ll still have to farm the gear yourself.
The microtransactions themselves only offer cosmetics, consumables (that aren’t more powerful than the craftable options), and utility stuff (race-change tokens, and skipping some of the skillpoint grind).
There is a player market for exchanging Crowns (microtransaction currency) for gold, and another player market for buying raid clears with gold. Raiding (called Trials) offer some of the best PvE gear. You don’t need to buy clears if you’re skilled enough to run those Trials and can find groups to do it with you.
If you just want to enjoy the game without worrying about min-maxing your build, all the base-game and craftable options will do you just fine.