Okay, let me check my understanding...

1)The car that I bought
Satin aldığım araba

2)The car (that is) on the road
Yoldaki araba

How about "Yolda olduğu araba"? Possible? Improbable? Ridiculous?

Should I always use "ki" for relative clauses without verbs (as in example 2)?
Can I use it for relative clauses with verbs (as in example 1)?

Wait, wait, found an answer here.

Greetings,
You got it right. -ki is only used as a relative pronoun and -dik is only used as a verbal adjective or verbal adverb.
In other words, -dik only applies to verbs and ki never applies to verbs.

Here are examples:

araba > arabada > arabadaki
car > in the car > the x in the car

sen > senin > seninki
you > your > yours

sabah > sabahki
morning > the x in the morning

yol > yolda > yoldaki
road > on the road > the x on the road

görmek > gördüğüm
to see > the x that I saw

bilmek > bildiğim
to know > the x I know

the -dik suffix also build adverbial clauses of time or reason when used together with 'zaman', 'için', -'den dolayı'.
"geldiğim zaman" : when I come
"gördüğüm için" : because I have seen

Notice that "gördüğüm" in "gördüğüm film" is different than "gördüğüm zaman". The former is a verbal adjective and the latter is a verbal adverb. So it has two usages.

Cool, çok sağ ol.