Also, to what extent should I restrict my visits to the UK during those 5 full tax years? Would the 90 day average rule apply?
This came up in the Robert Gaines-Cooper case. The day count demonstrates continuing non-residence. You have to establish non-residence in the first place. An important principle is severing ties with the UK, so owning UK property will not help you at all.
You would need to be sure that the CGT rules in your intended country of residence suit you. For instance, that split year treatment in the year of arrival is possible if that country (like the UK) charges CGT in respect of disposals in a (full) tax year any part of which you are resident there.
BTW, Belgium does not have CGT. It is not the loveliest EU country, but an abundance of beer, chocolate and chips with mayonnaise might sway you ...