Once you start working on projects.. you'll discover things you need. I can just list some nice to have items that are often useful:
- Jewelers magnifying glass.. electronics tend to be small and it sometimes helps to be able to see the markings on a chip.. or inspect a soldering joint closer.
- A multimeter with a microamps and microvolts ranges. 2-3 multimeters never hurts.. and now days the choice is plentiful. If you don't have an oscilloscope Min/Max features are handy and so is a fast updating bar graph.
- Current limiting power supply. You already have some power supplies so I wouldn't concentrate on this, but being able to limit current to say a few milliamps can save you from frying too many components.
- Oscilloscope. It's really hard to describe this versatile tool in one sentence. But let's just say, it lets you see what really happens with your signals and power rails. Without it you can be blind to fast changing events, which many electronics depend on.
- Logic Analyser. Complements the Oscilloscope. Lets you decode many digital signals at once. Can be fairly cheap and are essential when working with buses like SPI and i2c..
- Soldering iron (you mentioned you have one).. you need solder and flux, as well. Soldering fumes are toxic.. fume extractors can be expensive so just make sure you at least do your soldering in a well ventilated area. Or you can give one of these a try:
https://www.amazon.com/Aoyue-Benchtop-Solder-Smoke-Absorber/dp/B001RLZGKK- Hot Air rework station. Not required, but can make working with SMD components much easier.
- Arbitrary Waveform Generator / Signal Generator. If you end up working on your own circuits.. being able to generate just the signal you need at a push of a few buttons can be very handy.
- DC Load. Not really required. You can build a dummy load yourself as well. But there are some really nice programmable DC loads you can get as well. Say your testing a voltage regulator circuit you made. It would be nice to be able to test it under load, a DC load lets you do just that.
- Breadboards.. you can never have enough of breadboards imo. Great for prototyping. Also jumper wires.
- Jellybean components.. 100, 1k, 10K resistors.. capacitors, from small Ceramic/MLCC pF, and nF.. to a few bigger uF electrolytic capacitors. 2N3904 and 2N3906 NPN and PNP transistors.. some TIP120 and FQP30N06L n-channel MOSFETs.. some LM358 Op Amps and a few LM339 comparators.