Least difficult route driving from Milan to Tuscany

Here is a key to the numbering system for roads in Italy:
https://sites.google.com/site/roadnumberingsystems/home/countries/italy

Working your way from top to bottom on the chart, the E (European) roads and A (Motorway/Autostrada) roads will be the widest, clearly marked, well-maintained roads. Further down are the SS (Strada statale, state "highways") and SR (Strada regionale, regional "roads.) The further down the list you go, the more challenging the driving may become.

I've driven the E80 from La Spezia to Pisa. It's just fine.

I've driven some of the SRxxx roads from Lucca to Vinci. Many of those smaller mountain roads do not have lines demarking the lanes - because they are not wide enough for two lanes. If a bus comes by, be prepared for some mountainside maneuvering and backing up. Watch out too, as happened during my drive, for gaggles of bicyclists either racing or training on the hilly SR roads.

Based on the numbering system along - and nothing else (i.e., not based on distance) - I would choose your route #2 as the "easiest" driving.

Further down the road numbering system, there is a section called "Suffixes of SS and SP numbers." One of those suffixes is "dir" (diramazione, branch/spur.)

Your route #3 goes via SR 439 to Volterra, but it does not show SR68 in that route. That may mean that your route #3 is taking you via SR439dir. I've not driven SR 439/dir or SR68. SR 439dir looks like a much smaller regional road then SR68, though perhaps with fewer big switchbacks. Scenic though it may be, SR439dir may be more adventurous driving.

I should also say that the "adventurous" driving in the Vinci/Volterra region is spectacularly lovely....if you allow yourself plenty of time to your destination. If you're in a rush, those roads will be nothing but harrowing.