This article includes sales estimates for different handhelds from market research firm IDC.
They place total handheld PC sales of the Steam Deck, RoG Ally, Lenovo Legion Go, and MSI Claw at almost 6 million units for the past 3 years. It’s estimated that the Steam Deck makes up between 3.7 to 4 million of those sales, more than all the other major handheld PC manufacturers combined.
The only catch there is that installing mods can be tricky depending on the game and method. Steam workshop mods and games with integrated mod support (BG3, DRG, etc) are super easy to install mods for, but the mod installation process for many other games can vary on difficulty.
Hopefully this will all change with with the rise of more official Linux mod loaders like the new Nexus mods app.