And there are no general stacking rules. The rules you quote is only for magical effects, i.e. spells.
There is a new stacking rule in the DMG however which clarifies that other game effects with the same name don't stack. I don't know if there is also explained that you add together.
Common sense and a little math would make it 6x your speed. I can however understand that you feel more comfortable using the 3e math of adding the increases which is a good way to handle it.
Btw. How do you handle dashing with the monk speed bonus, or the monk speed bonus and haste, or haste and dashing?
Where is the section you are thinking about in the DMG?
For the jump discussion the section on Combining Magical Effects, while it explicitly refers to spells, seems to cover magical effects in general as other magical effects such as Paladin auras of the same kind only use the highest bonus. The
jump spell, being a spell, is explicitly magical. However, the monk's ki is also magical. It is the harnessing of mystical energy, powers the monk's attacks to make them magical, and magically keeps the monk free of disease. As both effects affect the same thing, the effects add together.
Extending that to the monk's speed,
haste, and Dash, there are a couple options. Most monks at 2nd level have a 40 foot speed. I do not consider the flat bonus to speed to be magical as several races and classes have this, and it does not seem to be magical in nature for any of them. With a Dash, the monk may move another 40 feet.
1. If the monk uses a ki for Step of the Wind, they can Dash for another 40 feet for 120 feet of movement.
2. If the monk has
haste, their speed is 80 feet. They can then Dash for another 80 feet. The question comes when the two magical effects combine.
2.a. The bonus action Dash from Step of the Wind could be considered to clash with
haste. In that case, the magical effect of the bonus action Dash is another 40 feet as it does not combine with the
haste, and the total movement would be 200 feet. This starts to get a bit weird as to where the combination happens in my opinion.
2.b. It can also be considered that the Step of the Wind and
haste are not affected by the magical effect combining rules because the give different effects. The spell
haste doubles speed and Step of the Wind gives a bonus action Dash. In that case, the bonus action is 80 feet, and the total movement available on that turn is 240 feet. This is the interpretation I follow.
