Hi, Helle,
I have walked both from LePuy, starting in June, and Sevilla starting in mid April and once in early May. I think you have correctly identified the pros and cons of LePuy route. I thought it was very beautiful, but there are more ups and downs. But if you do it, try to squeeze in the time to take the Cele Valley detour, it's an absolutely amazing route. The French guidebook has it well mapped out and documented. I have heard from many that your ideas about reservations in May on LePuy are accurate. There are lots of French who take two week walking holidays along this GR in May. We did not meet too many Santiago-bound pilgrims between LePuy and SJPP, though there were of course some.
Starting in early May from Sevilla is, IMO, not at all monotonous or boring. The dehesas (rocky fields) in Extremadura in springtime are gorgeous, tons of flowers, lots of streams, huge rocks, scrub oak, it's a very beautiful landscape. There are rolling ups and downs and a few short steep climbs but nothing sustained or major. The route passes through a lot of great places to visit -- Sevilla, Merida, Caceres, Salamanca, Zamora, Ourense -- and those are just the major cities!
I don't know your comfort zone for kms, but if you're ok with 30, there aren't too many that have to be longer than that. I posted my stages here
https://www.caminodesantiago.me/community/threads/my-stages-on-the-vdlp.8961/ and there really wasn't any that I'd describe as a killer stage.
Both times on the Vdlp, I had to ford a few arroyos/streams, requiring me to take off boots, put on chacos, and roll up pants. Nothing major or dangerous, kind of fun actually. There is one spot where there can be bad flooding, soon after Italica on day 1 before Guillena, but a detour will be posted if the arroyo is too deep (and rumor has it that they will soon be building, or are building, a bridge over that one spot). I had great weather on the Vdlp, a few rainy days, but lots of beautiful temps and sunnty skies. I think May is just about perfect on the Vdlp. There were other pilgrims, never a crowd, no bed races, no rushing, no reservations ever.
I think both routes are wonderful, but I am partial to the Vdlp! Buen camino, Laurie