Hello there! After years of being a enthusiast for photography, i now have a (limited) budget to get myself a dslr camera. İ am now in a dilemma, could go get a 250d with 18-55 stm brand new or a used canon 6d. İ plan to get 50mm 1.8 lens straightaway no matter which camera. My ultimate focus will be astrophotography but i'll also do portraits, scenes and street photography. İ'll also be happy to hear about your budget friendly lens recommendations.
An advantage to the SL3 is that there are more inexpensive lens available for it (the Canon EF-S series of lenses). Plus it will come with a lens right now. If you get the 6D you won't be able to use it until you get the 50mm.
When you say "astrophotography" are you planning deep space or star trails/Milky Way? For the latter a popular lens is the Irix 15mm. I use that for my shoots.
If your budget allows consider the 6D plus a refurb version of the Canon 24-105 STM lens.
Go with John's first recommendation. The SL3 is more versatile. Although I would typically choose FF, the SL3 beats the 6D in several key categories:
SL3 is 3 yrs old, vs 6D 8yrs old.
Your intended use, the SL3 is a wiser investment between these two. It has a Canon warranty as well.
Given those two choices, the SL3 is probably a better choice for astrophotography on a telescope. The full frame 6D is probably the better choice for portraits and street photography because of its better low light capabilities.
But, a good argument could be made for using the smaller camera for street photography. Sometimes you do not want to look like Joe “The Pro” Photographer..
You may also find better choices with a slightly bigger budget, too.