When heating up a pizza, edges burn while middle still liquid

The key here is that you said you’ve thawed the pizza. Frozen pizzas are designed to be baked from frozen. The instructions on the box should reflect this.

How big is the pizza in relation to the toaster-oven? If it takes up most of the interior area I could see this possibly happening. Have you considered cutting the pizza into smaller dimensions to see if you get a more even cook?

In general, when the edges are burning (or even just overcooking) while the inside is undercooked or raw, you can usually improve things by cooking at a lower temperature for longer.

A higher temperature means a higher temperature gradient, you see, because ovens/heating elements are always hotter than the food (to get things up to temp quicker), and it takes time for the food to come up to temperature from the outside in, so the higher temps will give more of a contrast, and lower temps have time for a more even heating.

You'd need to do some experimenting to get things right, ovens, dishes, and preferences all differ, but you can start moving the temp down (in F I'd suggest going by 50 degree increments, in C maybe by 20 degrees?) and start checking at the same time it used to be done, you'd only need to keep an eye on it a few times till you start to get an idea of how long it will take.

Another technique, a bit more limited but still effective, is to turn off the oven just a little early, and letting the pizza sit in the heated oven a little longer - effectively letting the food cook in the indirect leftover heat instead of using the directional heating from the oven elements. You can actually get quite a bit of cooking done this way, especially if you've a large oven, or a heat sink like a baking stone, but because the effects are finite it's best to use this for fairly minor tweaking. (I could for example, get a pizza to go from kinda watery to deep brown, smooth&dry [yes, a bit much], or cook three tortilla-base pizzas in the time between turning an oven off and when it cools - though as I said, that is my large and strong oven with a baking stone).

And third technique, if you're mostly happy with the outcome of the pizza base and it's just the toppings that are a bit watery or pale, is use a broiling or top oven element for a bit right at the end of cooking. The heating will be surface-stuff, especially in the middle (ie, won't make the insides hot), but it will work to give some nice browning or surface drying or toppings-cooking if that's your primary complaint.