Hi,
for several years I happily used SGS5 mini, also on bicycle, also in summer.
This worked great, no heat probelms.
(I am using Osmand mainly for following predefined tracks, not for navigation.)
Now I have a Nokia 8, which also sometimes has problems of overheating.
Therefor I am currently experimenting with night mode, high contrast map representation and with dimmer apps:
I am trying to find a way to easily toggle off (or dark) the screen whenever I do not need it and toggle it on again when needed.
(All that while riding the bicycle, while the phone is fixed within its mount.)
With my current experiences, I would recommend a phone which has a rather small but bright screen, and which has OLED (for less power consumption).