Lightweight tent recommendations?

Pitch it then paint seams from outside. Easy enough. See Six moon designs for a guide.

Consider the six moon designs, lunar solo, it's silpoly not silnylon so doesn't sag when wet. More money though.

Are there others? Lanshan is the name of the product of 3F. This interesting though brief read was linked here a few pages ago: https://www.backpacker.com/gear/chinese-ultralight-backpacking-gear/

I just bought 3F branded Lanshan Pro and spent my first 3 nights in it. I do own a nice 2p MSR Hubba Hubba nx which I would always bring when I am with my partner. Hands down the best tent I ever owned.
The Lanshan is lighter and more compact though for when I am on my own and since its kind of modular you can be a weight weenie and get the nicest and smallest carbon pole and ti pegs for it.

Doing a functional seam seal is very straightforward and easy. When I ordered mine it actually came with a syringe, brush and sealant to do the job. What is a bit tricky is to do a visually neat job. If you aren't bothered by looks you can do it in half an hour.
I followed these instructions: https://www.youtube.com/watch?v=3sv2Hhrn1Vg

Then a friend sent this: https://www.youtube.com/watch?v=4Tb0RxnQXoU
which is potentially cleaner and better looking.

Additional notes:

1. Getting a good pitch on this tent requires setting it up a few times. You need to get the tension right on all corners so the bathtub of the floor can do its job and it doesn't sag as much when getting wet or cold.
   
2. It's a single-wall tent so there is no layer between you and the outer wall that catches condensation. I am 6ft and there was enough space for me to not touch the walls when the tent was pitched right. I recommend bringing a tiny micro towel so you can wipe the inner. If the weather allows it just sleep with the vestibule open and condensation shouldn't be an issue.
   
3. It's not freestanding. So pick your spot well. If you want to move it you have to take the whole thing down. If the ground is too hard or too loose for your pegs it can be difficult to get a good pitch.
   
4. I also immediately replaced the floor/bathtub loops with bungeecoord which made pitching easier in my opinion.
   
5. Thinking about ditching the stuff sack as it is super slippery which makes it hard to get it to pack small. I actually ended up just stuffing the whole tent in my frame bag.
   
   

All in I am quite happy for the price/value ratio. The Lanshan will replace my Snugpack stratosphere hooped bivy as it packs down as small, is even lighter and provides more comfort.