[RPG] Do skill DCs have to be multiples of 5

You are misunderstanding a vital element of the rules. There are not two different kind of saves.

What you're reading as "Spell Save" is actually "Spell Save DC". It is used to calculate the save difficulty of spells casts by your character. Spell save difficulties are not set by the DM.

When you cast a spell that calls for the target to make a saving throw, the DC is equal to 8 + your proficiency modifier + the appropriate ability modifier. The target of the spell then makes a saving throw against that DC.

For example, a level one Wizard with 16 intelligence casts Burning Hands at a target. This would have a spell save DC of 8 + 2 (proficiency) + 3 (ability modifier) = 13. Therefore, the target must make a dexterity saving throw against a DC of 13 or take the full effects of the spell.

All saving throws are made by rolling d20 + the appropriate ability modifier, versus a DC. If the target has proficiency in the appropriate save, they also add their proficiency bonus. So, in this case, if the target were a level one Rogue with 16 dexterity and proficiency in dexterity saves, they would roll d20 + 2 (proficiency) + 3 (ability modifier). If the results of that roll equal or exceed the DC of 13, the save succeeds.

Yes, you can choose as you like.

The advice given on how to create new monsters in the DMG is a set of guidelines to help you get in the right ballpark, not hard-and-fast rules about how all creatures must work. You can choose whatever values you want based on how challenging you're trying to make the creature.

Most of the time (but not always) in the published material, the DCs for abilities are derived using the 8 + Proficiency + Relevant Modifier formula, so for consistency's sake sticking to that formula is neatest. I think the DC values as given on the Challenge Rating table are mainly intended for putting things together very quickly so they're roughly acceptable, and they look like values you'd get if you were creating a creature with a reasonably good, but not exceptional, relevant ability score modifier. In your specific case, you're making a monster with a truly exceptional ability score, so it's not surprising that it looks very different to the quick approximation guideline.