I wrote this a while ago with the intention of sharing, but then forgot about it. So this is my end of year gift to the community.
This is not a perfect solution but the best I can come up with.
Anyone with better ideas, especially how to get ḷ, please let me know and I will amend this.
To add these keyboards, go to settings> general > keyboards
Software keyboard (on screen)
Software keyboards are listed under the globe button on your screen keyboard (where all the emojis hide).
- Hindi (Latin), on older iOS Hinglish
This allows all characters except perfect nasal m’s - only m̐
Retroflex l (ḷ ළ්) doesn’t exist on the Hindi keyboard
Hold down t to get ṭ (same for ḍ and ṅ ṇ ñ)
Hold down a to get ā (same for ī and ū)
Hold down m to get m̐ (you could find and replace later if you wish, from a pasted ṁ)
Hardware Keyboard
(Using a Bluetooth keyboard- must be turned on to see in settings)
- Hindi (Latin) - ABC -Extended (This is also the default on English -Australian)
Option+w - nasal dot - ṅ ṁ
Option+n - glide - ñ
Option+h - retroflex dot (kinda) - ṯ ṉ ḏ ḻ
Option+a - long vowel - ā ī ū
If you’re typing in an app like pages you can do a find and replace at the end of your session to find ṯ and replace it with ṭ etc.
To type the ṭ (true retroflex dot) tap in the replace box, then tap the keyboard icon in the bottom right corner. Make sure your software keyboard is set to Hindi (Latin) and hold the correct letter down (ie t) to find it’s retroflex friend.
The retroflex l workaround - ḷ ළ්
Luckily ḷ isn’t used frequently.
Go to settings> Keyboards > text replacement
Tap + for a new replacement
Copy a ḷ from some pali text ie Buddha’s Words ‘oḷārikā’ and paste it in the phrase box.
Type l* in the shortcut box.
When you want to type a ḷ - type l* preceded by a space.
You would type o l* ārikā which would then convert to o ḷ ārikā and you have to delete the spaces out. Totally a ‘coarse’ solution 
.
If there are words you would type regularly with a ‘ḷ’, then once you have set up the above shortcut, make an extra shortcut for that word.
Example:
Type o l* arika in the phrase box (or copy from Pāḷī text) then fix it to oḷārikā
Set the shortcut as olarika
Frequently used words (with diacritics)
Follow the above method to save handy shortcuts for words you might type frequently.
For example: I have SD set to replace with Saṅghādisesa and ditthi for diṭṭhi.
If I don’t want SD to replace with Saṅghādisesa then I hit the ESC key on my Bluetooth keyboard or tap it on my screen. So I can still also write about SD cards 
I’m lazy so I have other Dhamma terms as shortcuts such as ‘6SB’ types ‘6 sense bases’
4 foundations of mindfulness ‘4FM” etc.
