| tmncx0 |
The Ready action reads:
You prepare to use an action that will occur outside your turn. Choose a single action you can use and designate a trigger. Your turn then ends. If the trigger you choose occurs before the start of your next turn, you can use the chosen action as a reaction (provided you still meet the requirements to use it).
-snip-
(Emphasis mine)
This explicitly only allows readying a single action, which precludes readying a spell. The rules for the Cast a Spell activity also seem to actively prevent any kind of readying shenanigans, where you partially cast a spell during your turn, then ready the final spellcasting action using the Ready action.
You Cast a Spell you know or have prepared. Casting a Spell is a special activity that takes a variable number of actions depending on the spell, as listed in each spell’s stat block. You can spend those actions in any order you wish, provided you do so consecutively on a single turn. As soon as all spellcasting actions are complete, the spell effect occurs.
-snip-
(Emphasis mine)
The bolded text makes it impossible to spread out the casting of a spell using the Ready action. The removal of this text would allow the readying of a spell, by allowing the completion of the final spellcasting action using the Ready action. Additionally, it would allow multiple-round casting duration spells to be cast during combat - and also allow the stacking of metamagic feats by allowing a spell's casting to extend to multiple turns by adding multiple metamagic casting actions to a single spell casting.
To me, none of these changes seem unreasonable, though it's possible there was an explicit design intent to prevent this. Either way, I've had multiple players express frustration when they are not able to ready spells by RAW. It's definitely something players expect to be able to do, especially because it was possible to do so in PF1e. Unless, of course, I've missed a rule somewhere that does allow this?