When you are in fastboot mode you can directly start the recovery image without installing it:
fastboot boot recovery.img
Alternatively if you want to start the installed recovery rom execute
fastboot flash recovery recovery.img
fastboot reboot bootloader
and then select Recovery using Vol-Up/Down buttons + Power button to start the recovery ROM.
This is a workaround and not what the OP wants in terms of vertical recents ( I don't think it's possible for reasons mentioned at the end of answer).
Install EdXposed (How to install EdXposed on Android 10 without triggering safety net), Xposed Edge Pro at a minimum and Gravity Box for 10 (beta) (preferred).
In Xposed Edge Pro, enable gestures and select ...