These 3 bikes are very different!

First the Dahon MU P24 is very similar to your Tern Link P9, its heavier, the 3 speed rear hub is heavy and has a lower efficiency than a derailleur only transmission, it won't ride better than your Link P9. It also folds similarly with a similar folded size.

The Tern Eclipse P18 has bigger 24" wheels with all the problems of finding efficient tires in that size (same problem as Helix) for this reason, Tern moved to 26" wheels on the current Eclipse. It folds similarly but bigger than your current Link.

The Birdy Touring is a totally different type of bike with a full suspension and a different, smaller fold. There were several versions of the Riese & Müller Birdy Touring, Birdy 2 and Birdy 3 with different transmission including one with a SRAM Dual Drive 3 speed hub discontinuated by SRAM. The Birdy rides great on all type of roads and trails.