I finally gave in and bought an oscilloscope, the Siglent SDS1104X-E.
The main excuse was that I’m doing more hardware work these days. This unit from Siglent is near the bottom of their range, but still packs in a ridiculous amount of functionality. I’ve watched as compact LCD models gradually took over from the cathode ray tubes I remember from school. Of course you can remove the hardware element almost completely and buy a PC dongle, but you miss the ergonomics of a physical unit.
This unit includes serial decoding for SPI, I2C and others, and I’ll be testing those features with the new Raspberry Pi gear I bought recently.
To go along with the scope I bought a cheap signal generator - a FeelElec FY6900.
This is a 60MHz dual channel device with 250MSa/s at 14bits resolution, and available from Banggood for around $100. I’m not qualified to comment on its performance, but it’s very handy for sanity checking.