I think this might be techinically true. An action chosen by Ready occurs when triggered (and you use your reaction for it). In the vast majority of cases this will probably be on someone else's turn*. This means that Dash and Disengage are pointless to use with Ready.
And while the technical details are a bit nuts, I think it is the RAI after all: Ready is an action like others, so you can already Ready+move
on your turn, so if you could obtain a double-move by readying Dash, you would end up with a triple movement in the course of the round (turn + reaction).
*You can fabricate a case when for some reason the player wants to Ready + move (instead of the more reasonable move + Ready) and the move triggers someone else's reaction which in turn triggers your Ready action, but I think it's quite artificial
For different reasons, Dodge is also pointless to use with Ready (it works, but it uses your reaction and the benefits starts
after the trigger, so it makes no sense compared to just taking Dodge on your turn, which makes it last longer and doesn't consume your reaction) and Cast a Spell
could be useful in extremely specific circumstances, but in most cases it's nonsense, thanks to the ad-hoc rule for readying spells which makes you waste the slot even if you don't cast it. I rather question the RAI for this one, as IMHO it's overly complicated and makes ready a spell nearly useless, compared to just allowind Ready the "Cast a Spell" action as normal.
Technically yes, but you then need a
second reaction to take the second-readied action, since you use the first reaction to take Ready a second time. I think it's quite obvious that even if your PC for some special reason can take 2 reactions, it makes no sense to make this choice.
Now, this thread made me want to see what happens if I play a Rogue(Thief) 17 / Fighter 1 with the Magic Initiate feat, and then use Thief's Reflexes in the first round of combat (to get 2 full turns in the same round including 2 actions + 2 bonus actions + 2 reactions), use the 2 bonus actions for the Expeditious Retreat spell (which grants... Dash!), activate Action Surge (such a pity I can't get the second use of it) to get a third action, and blow at least some of those actions to Ready something else