There is little chance of bricking the SSA3021X during a firmware upgrade, especially if you already are at v8.x.
It would take extra effort to deliberatly brick it - it would be easier to fry the input stage.
That seems to me as a totally ridiculous demand of the seller: if there was any risk, then Siglent themselfes would not offer a public firmware upgrade. It would be done by authorised centres, like what is done in some cases with premium brands.
Also, tell your seller that the warranty is covered by Siglent, so if the upgrade would brick the device, then Siglent would have to repair it.
It makes no sense at all using a SSA3021X with an older firmware, especially if you encounter bugs which have been fixed in newer FW releases.
Regarding the manuals, it is always complicated to keep up the manual, when developing the firmware. Yes, they should finish it and keep it updated along with the firmware releases, but the truth is, that the device is pretty self-explanatory. I never fully read the manual. I guess using the device just makes sense.
Regards,
Vitor