Which espresso accessories actually improve the coffee?
Three change the cup: a 0.1 g scale, a WDT tool with 0.3 to 0.4 mm needles, and a precision basket. Almost everything else is convenience or decoration.
A scale is first because it makes everything else measurable. Weighing the dose in and the espresso out turns a bad shot from a mystery into a diagnosable problem. For espresso you want 0.1 g resolution, a refresh rate fast enough to keep up with a pour running at 2 to 3 g per second, and a body low enough to fit under your portafilter. A timer and auto-tare are the features that separate a coffee scale from a kitchen scale.
A WDT tool is second because it addresses channeling, the defect that ruins the most shots. Needles between 0.3 and 0.4 mm slip between grounds and break clumps; anything 0.5 mm and thicker starts plowing furrows. Five to eight needles is plenty. A weighted stand that fixes the depth makes your prep repeatable, which is the actual goal. Beyond needle gauge and a stable stand, nothing on a WDT tool affects the coffee.
A precision basket is third. Stock baskets that ship with entry machines often have uneven hole patterns and thick, tapered walls, which distribute water unevenly and encourage edge channeling. A precision basket costs little and gives you a straight-walled chamber with a consistent, laser-drilled hole pattern. Buy the one that matches your dose, since an 18 g basket run at 14 g will underperform.
Useful but not transformative: a dosing funnel, which mostly stops mess and makes stirring practical; a bottomless portafilter, which does not improve the shot but shows you exactly where your prep is failing, which is worth a lot while you learn; a puck screen, which keeps the shower screen cleaner and can help distribute incoming water; and a decent tamper that fits your basket properly, which matters mainly because a loose-fitting tamper leaves an unsealed ring.
Things that mostly do not change the cup: spinning distribution and levelling tools, which flatten the top surface without addressing density deeper down; bottomless portafilters with exotic handles; magnetic everything; and calibrated tampers, since tamp pressure matters far less than tamp levelness. Buy those because you enjoy them, not because you expect better coffee.