Just want to chime in and say this is the correct range that has been talked about for a few years for global production, so it's not only from one source - i've seen the same estimate from Ineos directly a few times and it was repeated at the in-person event in Calgary to see the Grenadier in the flesh.
for aftermarket stuff, I think you'll be impressed with the number of partners Ineos has reportedly worked with. I understand ARB, Ironman4x4, and a few others in Australia have already been enlisted for support and provided with the CAD files for making accessories (https://www.goauto.com.au/news/ineo...ns-developed-down-under/2021-09-27/85989.html
I've also read elsewhere that Alucab is working on a roof conversion (like the Icarus Defender or the Troopy conversions). As far as things like Roof Racks, I've seen both Rhino Racks and Front Runner racks in various media.