What do you think are the minimum requirements for a planet to be worth upgrading?

Terra-->Gaia (VL + multimega?)
Barren-->Oceanic (VL + rich?)
Desert-->crystal (VL + ex rich?)
Icy-->oceanic (VM?)
toxic-->irradiated (VL + 16x?)
gas-->plasma (VM + Multimega +1000 passive?)
volcanic-->demon (VL + Multimega +1000 passive?)

Plus any others I've forgotten.

toxic, volcanic and gas - max it out on everything first
desert, barren, icy and terra - up to you just remember you have limited amounts of them (same applies to toxics with that)

I would say VL, 15x-16x mega rich on everything. It's better to save your limited planet upgrades for later. I foolishly made the mistake of wasting my mass surface melters on Icy's that just weren't that good. (I won't make that mistake when I reset.)

Desert is max of VL VR, when turned Crystal it'll a M UR without uplifts.

terra/barren/desert/icy should all be 15x and very massive before you upgrade them. You only have a limited number of the items to upgrade them. Size matters more than richness in a ratio of about 5:1 basically based on drop rate. Biggest complaint of high ranking players is they wasted their melters when they were still noobs and cant get any more. Mega colossal makes planet building that much more worthwhile.

gas--VM
others VL
toxic 16x
others 15x

actually, u can probably trade 5 melters + 3 15x VL icies for 3 17x MC oceanics, basically trading 2 melters for 4 QSE.
thou being planet trade, it will be a hassle, and might cost GP in abondoning planet (otherwise this will require prior arrangement OR saving up a lot of terraformers etc)
In the long run, if u don't plan to buy GP, I would suggest such trades if able.

personally I am using all my planet transformers on 20x arts. And I mean all, sentient and hydroshifters too, starting with vl planets to save on QSEs. I also plan to make at least of of the other resources maxed out. Ie X/22x/17x Mc oceanics and 18x/22x/X irradiated. It will take awhile, but I am in it for the long haul. Also any with stealth shields is worth upgrading size not being the issue.

How many crystals have you obtained so far? or are you hoping someone will be selling their trelith resequencers?

Wow either way good luck with that. 225 crystals or resequencers needed for that assuming you havent already used some on unmaxed planets.

I have used 5 resequencers and gotten 8 crystals. I maxed out a C crystal to 20x and my exotic. I have a gaia that is 18x. I haven't and a MC oceanic at 15x, I haven't done the melter mission once =) but I have a icy I am preparing to use my resequencers on, just need another 18 terraformers. Should be another couple months plenty of time to get a couple more crystals to max out the gaia before I start saving them for the next planet.