How many labor hours should it take to replace the starter on my 2000 LS400, given you have to remove the intake manifolds? Should a weekend warrior attempt it themselves?

Intake manifolds are not difficult. Simply remove and fold back what is in the way, undo bolts and nuts until it comes off. If you are careful, you may not need to replace the gasket in between. The difficult part of doing a starter is that they are heavy (for one hand in a weird position) and they are difficult to manuver into place. Also there is a very small nut and wire that sometimes strip off after you have connected them when you drop the starter and that little wire is the only thing catching it.
Maybe 3 hours tops professionally.
Here's what you do. Simply go for it slowly. Have a friend with you to bounce ideas off with so you think through each procedure carefully. A helpless friend can be more help than you think. If you think this is getting too involved for your skill level, reverse process and put it back together. If you can't get it out in 4 hours, put it back together and hire someone. No shame in that. Find an old shop where the mechanic is over 50 yrs old. They like to teach. They even might let you watch, depending on how sophisticated the shop is.
I once took my truck to some 25 yr old with glasses, pocket protector with pens, screwdrivers and a couple of flashlights attached at the belt. If his computer could not tell him what was wrong, he was at a loss. I then took my truck to a bent over old mechanic who could no longer stand up straight. He walked me through the mind set and procedure and I then did it myself, with his guidence. Not the typical shop, I agree. But I learned more from that old mechanic than these "Johnny on the spot" mechanics who think they have it all figured out.

Slowly attempting a procedure and backing out is how we learn. Just don't go deeper than you can deal with. Don't cut anything either, like wires. All wires disconnect or move out of the way.

At my dealership we charged 8.0hrs for the starter. The easiest way to do it is if you use a lift to get to the 2 starter bolts on the back of the bell housing. I've done a couple of them and in a shop I could do it in about 2.5 hours. The ls400 is the hardest starter on the lexus', there's just no room in the engine bay. it's a kinda big commitment but if you think you can, then why not try it? hoped this helped your decision.