I have always good results when transplanting chili, tomato and other nightshade seedlings very early. You need to handle them carefully though and the success somewhat depends on your fine motor skills. The actual nutrition plan is important as well - if you transplant early, it doesn't mean that you should expose them to full concentration that early. Actually I grow my seedlings with quite high concentration in the first place (which isn't by the book) and hence I can switch to full concentration nutrients much earlier. It's the extreme change that may stress or kill the seedlings, not the actual concentration. You may also trim extra long roots, to encourage lateral root growth.

Low nutrient concentration (or imbalance) as often recommended for seedlings may cause disproportional root growth compared to sprout size. That isn't what one aims for and I use up to 1.3 EC for tomato seedlings from germination but with a special formula.

Remark: I have tropical climate and kinda ideal conditions at my location - what is true in Naples, mustn't necessarily be the case in Sevastopol...

I've been fertilizing once a week with 1/4 strength GH MaxiGro for the past two weeks, but I don't know if this helps the diagnosis of my case.

As for trimming roots, what sort of guidelines should I use in choosing what should be trimmed? How much to trim off? Any other special tips or advice?

Is it possible that you were using too small cups and lateral growth of roots was limited by this simple fact? One can anyway deduce from large root size (compared to sprout size) that nutrition requirements weren't sufficiently covered. Next time use bigger cups for the seedlings (in case they were rather small) ad increase nutrient strength. When transplanting, trim 1/4 to 1/3 of total length (with sharp scissors).

But isn't there any decent "baby formula" around? Because feeding 1/4 strength with any "standard" product that is supposed to be used in much higher concentration can't be good. Simply because if you dilute it by 4, you obvious haven't got enough Phosphorus, Magnesium, Calcium and trace elements. Nitrogen and potassium may be sufficient, if ever - but all other elements come really short.

I'd at least use half strength in case I could not lay hand on a product that uses a special formula to be used in such low concentration.

Well, for someone who has no insight in the finer parts of nutrients and formulas it may not make much sense what I am telling here, but a quarter strength of any such formula is simply not appropriate to grow seedlings and younger tomato plants. Here's an example to illustrate what a decent tomato formula from seeding to transplant should actually look like;

N = 70 ppm
P = 50 ppm
K=120 ppm
Ca = 150 ppm
Mg = 40 ppm
S= 50 ppm

In this formula the adequate quantity of P, Ca, S, and Mg is respected - it's only notably lower in N and K. Now, if simply using 1/4 strength of any "standard product" you obviously end up with something like:

N = 50-60 ppm
P = 10-15 ppm
K = 70-80 ppm
Ca= 35-45 ppm
Mg= 10-12 ppm
S= 15-25 ppm

Even if I am generous, that's far from what is required, ain't it?
Not even mentioning only a quarter of the needed trace elements that you'll end up with! Because any decent nutrient should actually have the required amount of trace elements, not a quarter of it! With half strength you get closer to what is required, but still haven't got enough P, Mg, S, Ca and trace elements!

So if you really want a decent formula to grow tomato seedlings, try to lay hand on a product that has at least nearly the specs shown above (or uses a similar composition at low concentration).

Cheers
Lucas