I have a bookmark for http://geoportail.fr/ but it seems to be off line at the moment. When its working it has tools for measuring distances & areas & for plotting profiles, all on a selection of maps of various scales
Seems the URL has changed since I bookmarked it on this laptop (usually look at it on a desktop with a large screen). Try https://www.geoportail.gouv.fr/
I was going to suggest OpenRunner, but I haven't used it in a while and now the IGN tiles are only available if you have a premium account (30 euros / year). The free version has open-source maps available which might be good enough. It does a good job showing height gain, gradients, etc.
The site I linked to above lets you view IGN maps at various scales, plus aerial photos & some thematic maps (eg geology) on a PC/tablet but not to download them (although a screen shot is an option). Even if not much use outside, its good for planning.
You can choose the IGN Topo layer then draw your route. It then generates a profile with the elevation plotted.
You can even add way points with icons (parking, refuges, water stops, cols etc) and notes and this all gets generated on a printable route card. Eg https://tracedetrail.com/fr/trace/trace/149583