gear advice for soloing the John Muir trail in the summertime

Check the gear list subforum. You'll find dozens and dozens of threads from prospective JMT hikers this year alone.

It also helps us to have more direct questions.

Take a 5 day trip, whatever you carry on that is all you need, except maybe some special stuff for high passes depending on time of year you go.

If you haven't done any real trips, it's unclear if that's the case, then start with a 3 day trip. That helps show what doesn't work well, without making you commit to using bad gear for more than 2 nights.

A five day trip from my experience differs only from a 7-9 day no resupply trip in the amount of food you carry. So if you haven't taken a five day trip, you need to get started.

It's unclear to me how you can carry a 15-20 pound day pack, I'm not an UL backpacker, just L, light, and a twenty pound pack would be a 3 day trip with food etc, easily. The plus side is you are hiking already carrying the weight you will be carrying, which is actually great. I do that too on dayhikes, maybe carry about half the actual backpacking weight, because I don't use light gear on day hikes. Anotyher tip: if you are going to use trekking poles, use them on day hikes too, that way you train with them and get used to them.

Here's a short list:

Decent backpack, 2 pounds, give or take. Sized to fit your actual gear, including a bear cannister on jmt. Cannisters are big, so either carry it inside or outside, you have to know that before getting the pack.

Good sleeping bag, 1-1.5 pounds. Something like a thermarest prolite, 1 pound, cookset, complete, less than a pound, ditty bag, with medical stuff, less than a pound, tent, 2 pounds, tarptents are nice, get the one you like that fits your style. If you use trekking poles, by all means consider getting a trekking pole supported tent.

Layers and other stuff. It's really not that much to think about once you have the basics down, and really the only difference is the pack, for cannister carrying, and the amount of food you bring.

Short trips show you quickly what works and what doesn't, an overnighter for example will show you right away if a sleeping bag is ok, if your pad is ok, etc. Ie, practice a bit, it helps.