We're currently planning 4 nights in CT and 3 in Lucca next June. Should we put more of the nights into Lucca and less in CT?
Personally, I'd take at least one night from CT and one from Lucca an give them to somewhere else.
Are you an avid hiker, or do you have plans for day trips while in CT? Unless there for extensive hiking or complete r&r, you could shave a day from CT if you need it elsewhere.
Similar for Lucca, what activities do you have in mind? Opinions don't matter much, but they can help you sort out your own, so I'll just say that I don't share the widely held passion for Lucca--nothing wrong with it; it just didn't "do it" for me. Two full days would be sufficient to see everything in the town and maybe even a day trip. Let us know you what you had on your list to do while there and we can better advise.
I look at an overall itinerary as well--sometimes you need to balance out a trip with some time for laundry, etc., so I'd say we should see the whole plan.
We do like to hike, but also relax and just "take in" a place.
In Lucca, we thought we'd make a day trip to Pisa.
Our itinerary for 26 nights includes (at least at the moment) Milan, CT, Lucca, Bologna (or Parma), Verona, Dolomites and Varenna.
I mean more the distribution of days--do you need those days elsewhere, that kind of thing. Otherwise, it is just spitballing--I'd choose whatever calls you!
Looking over the hiking options might help you decide: https://www.parconazionale5terre.it/Esentieri-outdoor.php