There are apps available ( listed at end ) but IMO , automation is the preferred way for this since :
I prefer using Macrodroid automation app because it's free ( upto 5 macros ) and very easy to learn
You would need two macros for this
(Triggers / Actions / Constraints are added by +
on the right side of each panel )
Logic:
Ring on Calendar Notif Macro : Whenever you get a notification from your calendar app, alert you with a non - stop ringing , at a volume level and sound customised by you
Stop Ringing Macro : Manually stop the ringing using a widget on your home screen and set the Volume levels to what you would prefer in your normal device usage
Barebone implementation of this logic is implemented in Macros below. You can customise it later by experimenting

Don't get intimidated by the long explanation - it's detailed so that you don't waste time setting it up ; shouldn't take you more than 5 minutes to set up :)
Ring on Calendar Notif Macro
Trigger: Notification Received → Select your calendar app → Text → Any
Actions
Set Macrodroid Variable → New Variable → Stop Ringing , Type Boolean → Set it to false (initialising variable that controls when to stop / start ringing )
Repeat Actions ( second item on drop down when you press the plus sign on action panel ) → While condition repeat → Conditions +
→ Macrodroid Variable → Stop Ringing →OK
→ Select False and OK
This creates a while / do loop wherein we want you to be sued when notification arrives something to be done when . You see now a WHILE / DO - End Loop created . Tap the End Loop and you will get a pop up Add action above. Tap that and add actions below
Volume Change → select Media / Music and set slider to say maximum % tap OK
Play → select any sound from drop down list
Wait before next action → set it to 2 seconds
Constraints: Leave blank
Stop Ringing Macro
Actions
Constraints: Leave blank
Go to the widgets section of your launcher , drag and drop Macrodroid widget to your home screen and assign it to Stop Ringing macro
Alternative app option
With this simple app you can override or replace default reminder with "unmissable" ones - they will ring like phone call how long you want, with melody you want!