Yes, you can reinstall 0.83, or whatever; there’s no forward-only restriction on firmware installation.
For video modes, the PS2 will go as low as 240p, which is probably that 720×240 resolution you encountered. If you don’t want to deal with 240p, or your hardware doesn’t properly support it, you can have the OSSC line-double it to 480p.
As for OBS and 480i, I’m not sure why it’s collapsing 480i60 into 480p30 instead of deinterlacing to 480p60. Do you have 60fps output enabled in the Video options?
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