So I have a couple of VL Terras at 15X now.

Should I just use the Seeds, or should I just wait out getting E-Crystals and bump them to 20X first?

I know the second seems a more obvious choice, but I have terrible, terrible luck in getting Crystals

Patience is a virtue.

My strategy was to wait until I had 20x and then transformed the planet with my own devices.

If you're buying from someone else you may as well just use it ASAP unless it's like a VM Icy with SS and ACR. The extra production while you're waiting for crystals will far outweigh the benefits of waiting for that extra 2x.

So... wait if they're yours. Do it now if you're buying them.

Personally, I won't use gaia seed at all if I'm not going to invest in the game. I'd rather sell all my seeds, all my irradiator etc, all the limited planet transformers EXCEPT melters. And using the resource gotten from those sales, I'd buy VM icies which I will proceed to slowly make into 20x VM and finally 22x MC.

Why? because except a small advantage in starting production and therefore save a few talth ability or terraformer uplifts, a M 17x or even 22x ain't really better than a C 20x. And as it were, C 20x, plasma to be specific, can be mass produced.

This is also why the transformers except melters had a huge drop in value(they used to trade for a ton, much more than QSE at least). While they do have the potential to make C 21/22x which is slightly better than C20, the extra investment required(2QSE each) and the small difference they carry make them somewhat pointless.... Only MC 22x is sufficiently significant and prestigious in its own right to be considered different.

My approach is to split the difference. Some of my Seeds/Crystallizers/etc I use right away to get the production boost sooner, and others I hold onto in order to have the satisfaction of a few truly maxed planets.

It should only take about 50 level 6 base crates to get you 5 crystals.

Using a crystal on a very large planet is a bit of a waste though.
You probably won't make that much from a massive 22x
Would be better off selling every other set to buy QSE from someone and make yourself a nice colossal 22x

If you cant max cloak the planet I would give a +1 to waiting/selling.