Marty, mkirsch I think is right. Any of the plug n play 2” extensions are universally rated for 3500lbs. Afaik. That said how could an 8” extension not be stronger than a 24” extension of the same construction. The rating is likely a cya because the mfg doesn’t know what kind of receiver it’s going into (which is what will likely break first if overloaded, before the extension).
What you posted is a link to a Cannon or Super Truss extension. The OP would actually need a long extension similar to a Cannon in the bottom tube of a super hitch to even think about it working with that bike carrier thing. So on that front, good suggestion.
Regarding the universal 3500lb rating. Pffft. Towed an (estimated) 6500-7klb trailer with no lack of tongue weight from here to Anchorage during mud season, where all the frost heaves were still very much at their max. 18” “3500lb” extension hanging off a factory 2” receiver. Never questioned it and it never gave any problems.
