SWD refers to two different things: a light debugging protocol with clock and data, plus ground; the 2x10 0.05" connector usually used with it.
Flashing the Xiao board through USB often fails, with the board freezing. Using an external debugger solves this issue.
So now, I am using an external debugger also for flashing the board. The external debugger could be a Xiao SAMD turned debugger or, better, a Segger probe.
Segger used to list the Edu mini probe at an affordable price; it is no longer the case. Price went from USD20 to USD60.
The Xiao expansion board provided the pogo pins to make the connections, alas only for some boards, SAMD, nRF52840 and RP2040. The ESP32 requires full JTAG, except the lastest C-series which include the debugger.
I would trade the USB connector on the Xiao boards for a SWD or similar connector. There are various variants of the SWD connector, some with a 3V3 power line and UART RX/TX like the Silicon Labs Simplicity mini connector.
Raspberry Pi has done an excellent job with the debug probe and the fool-proof connector. It would be an opportunity for Seeed to release an affordable debugger probe. Seeed could come with its own connector, for example a 1x6 1mm for 3V3, ground, clock, data, TX and RX.