If you have troubles dividing words into syllables, just remember that you have to put as much consonants in the next syllable as possible.
scherps - te => wrong, a syllable can start with st
scherp - ste
scher - pste => wrong, a syllable can't start with pst
zi - ngen => wrong, syllables can't start with the ng sound
zing - en
ut - recht => wrong, a syllable can start with tr
u - trecht
You are probably aware that short a, e, i, o and u (like in pad, pet, pit, pot, put) are not allowed in open syllables in written Dutch, so when those sounds do occur in open syllables, the next consonant is doubled, for instance "latten" and "diffractie". That extra consonant is not pronounced.
Exception to all of this: Any vowel before "ch" is short. For instance "lachen" is with a short a.
"goochelen" and "loochenen" are the exception to this exception, and are spelled with double o to make it clear. This is quite unusual: goo- and loo- are open syllables, yet spelled with double o!
Is there any word starting with /x/ represented by "ch" or it's only the case of loanwords?
What about "th"? I have found the preceding vowels in some words are pronounced short vowels (kathedraal), in some are pronouced long ones (mathematica), in some both long and short ones are OK (atheïst, pathologie).
And are Mathijs and Matthijs pronounced the same?
Dutch words never start with /x/ besides some loanwords from Greek, so the spelling indeed makes sense. You could analyze lachen and goochelen as: lach - en, gooch - e - len
if you assume that syllables shouldn't start with ch.
It is common (especially in Flanders) to pronounce unstressed /a o e/ as /ɑ ɔ ə/.
You will definitely hear kathedraal, mathematica, atheïst and pathologie with short a, however, I think all of those words are with long a in Standard Dutch.
If Mathijs is Flemish, then his name is probably pronounced the same as Matthijs. If he is Dutch, then I am not so sure. I guess it should be with long a.