Hi, just some ideas for your consideration in route planning:
Panama- a loop around the south of the Azuero Peninsula, nice coastal route , sometimes a bit of gravel in construction zones, south off the PanAm highway and back again.
Watch out for the cops along the highways around Santiago in particular, they like to stand in the shade and nick you for the least little infraction, so stick to the speed limit and no passing in curves.
From David ride up the mountain to visit Boquete, interesting climate. From there or from David use the paved backroads to Gualaca and the road to the humid ,jungly north coast at Chiriqui Grande and west to Changuinola, then into Costa Rica at Sixaola - a very interesting crossing ,maybe a bit hairraising, via an old railroad bridge.
Costa Rica - the entire country! but especially a ride up to the top of Poas Volcano on a CLOUDLESS day, or at least with the hope of some fog free visibility. It is a whole different country than the warm lower slopes.And of course a ride along the Arenal Volcano and the Lago Arenal and over the top to Tilaran where, bang, teh climate switches to warm and dry Guanacaste.
Nicaragua- San Juan del Sur , Granada, Leon,
Honduras - Copan, Santa Rosa Copan and the gravel road west to / from Gracias and the PanAm.
Guatemala Antigua GT, Quezaltenango (Xela) and the road south through the valley along Zunil, the western highlands around Xela , Huehuetenango and around Lago Atitlan with a visit to Panajachel;
teh boat ride down the Rio Dulce from Fronteras on Highway CA13 down to Livingston , then to pretty Flores and Santa Elena ( lots of hotels in Sta. Elena off the island) and of course a visit to Tikal the impressive Mayan ruins site for a visit of one 2 or ??? days. Tikal has 3 hotels on site and a campground and small restaurants, so everything you need.
Belize - small enough to do a zigzag tour in a few days down to Punta Gorda and Dangriga before going north to Mexico.
Mexico- a word of caution - it is a big country and you will have to pick some route which fits your timetable, but as you are visiting in January you may want to keep a very close eye on the weather forecasts when you are getting into the northern parts. January is the coldest month ( now in late November they are already getting nights down to -4Celsious and lessin Tepehuanes and Temosachic , people suffering from the cold see www.tvazteca.com
, click on hechos/estados ) ) and if you rush into the highlands of Chihuahua to see the Copper Canyon you will get very cold ,even a good chance of snow, and I don't think you left Worms to see snow?If Nearing Durango from the cool centre of Mexico look if the weather to the north is worth the risk, then head to the Pacific coast along Mex 40 - thousands of curves, and to Mazatlan to either go north along the coast or across to Baja and then north-two very different countries but 100% interesting.
Five weeks should be good for the trip you propose , but will put a limit on how long you linger in any one spot. Enjoy !!