Hello Morgan and welcome to the forum. I'll second the route through Venosc and on up to La Bérarde; a good stop is at Venosc where the village up the hill is pretty and has craft shops plus local honey etc. Also, stop off at St Christophe on the way; we've always stopped there at the very least to look in the graveyard - many graves of climbers and guides and their families who have died on the mountains, overlooking such a lovely view too.
We've stayed in Venosc/Bourg D'Arud for many years and always drive up to La Bérarde, park and then walk as far as we are able, noting the number of new landslips and and enjoying seeing marmottes and other wildlife.
Col de Croix de Fer is an excellent idea, and we also like Col de Galibier. But a lot of people are put off by the drops; on our first visit to the area we were warned it was too dangerous to drive some of the routes, but found we'd met somewhat similar in the wilds of the Scottish Highlands, and since those days the route up to La Bérarde has had walls added along the way.
Hope you have a great trip!