Obviously your trip planning is very flexible ,which is good . If the Costa Rica goal was only because some family member is going there on a fly-in vacation it puts you in the bind of needing to BE there at a specific time . Schedules and deadlines are the bane of pleasure travel .
The seven day estimate for traversing Mexico ,Guatemala,Honduras and Nicaragua is possible ,but you would need to ride persistently during DAYLIGHT only of course . If you are a rider who likes to actually SEE a country you would want to slow it down .
Also ,that time frame was possible in the pre-covid era. Now there exist the possible and probable added delays at border crossings with the requirements for recent proof of Negative Covid tests.These may require some hunting around for test sites and extra paperwork to be processed- more time wasters . Though the C.A.countries are small they can require more time to cross than you may suspect. If you manage to cross one per day you would be doing well. Certainly for preservation of your happiness you would want to avoid doing TWO border crossings in ONE day.

The crossing of Mexico could be done in five days of steady directed riding and that would preclude any detailed sight seeing excursions or relaxed exploring of odd interesting roads and scenic places ."Northern Mexico " as starting point is also vague - Matamoros is a lot closer to Guatemala than is Mexicali .

Now that you imply that you could limit yourself to Mexico you have released a lot of the stress of needing to be someplace much farther away .Of course if you are starting from Canada in January the Fly-in thing
is understandable . .
Your idea for commercial trucking the bike south is sound .

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93