If you do your own chain/cassette maintenance and replacements, a chain tool, chain pliers, and cassette lockring tool are IMO "must haves". If you do your own wheel/hub service, IMO cone wrenches are a must if any of your wheels have cup&cone bearings (not needed if your hubs are sealed bearing designs). These are all relatively inexpensive, even if you go the higher quality route (Part Tool or Ice Toolz).

Depending on how experienced you are with mechanical work and judging torque by "feel", a set (1/4" and 3/8" drive) of inexpensive torque wenches might be a good buy as well - the 1/4" drive in particular if you work on carbon components. Ditto a set of metric Allen wrench drivers (sets of 13 ranging from 2.5mm to about 10/12mm are available) if you don't have them already. But a pair of even lower-end torque wrenches are going to cost you around $50 or more. The set of Allen wrench drivers can be had for <$20.

Many other bike specific tools exist, but you may or may not need them depending on the precise equipment you have.

Just my proverbial "two-cents worth".