I think the recommendations here are good for both VoiceOver and low vision users but please experiment to discover what works best for you.
iOS 18.1 brought Apple Intelligence to some recent phones in US English only. If your iPhone has Apple Intelligence the recommendations remain the same but you will find the settings for Siri in Settings / Apple Intelligence & Siri.
Those of you without Apple Intelligence will find Siri settings at Settings/Siri.
Under "Talk to Siri" choose whether you want the iPhone to listen for Siri or Hey Siri , just for Hey Siri or stop Siri listening for these action phrases. The choice to listen for Siri or Hey Siri may not be available on some iPhones. Select your preferred language and voice. Ensure that "Allow Siri When Locked" is turned on. Under "Siri Responses", select "Prefer Spoken Responses". If present here turn on "Always Show Siri Captions" and turn off "Always Show Request". Go back to Siri settings and change the call hangup setting if you wish. Under "Announce Calls" select "never". Calls will be announced by VoiceOver so an announcement from Siri is superfluous. Do the same for announce notifications. "My Information" is supplied by your Contact. If you haven't set up a contact for yourself, please do so before setting up Siri. You probably don't need to change anything on the remaining Siri setup pages but you might want to take a look at the Automatically Send Messages setting.
Note that if Hey Siri is not already set up then it will be necessary for the phone's user to speak "Hey Siri" and some other phrases several times to enable the phone to identify the user's voice. This process works well with VoiceOver since all the required phrases are spoken by the phone. Low vision users may find it helpful to turn on VoiceOver for this task since very large font sizes can make it impossible to read the requested phrases on screen.