Zeal is जील. In Hindi, ज is pronounced as the English 'j' like in 'jar'. In Marathi, ज is pronounced like the 'z' in zeal.
Also, it's not tamizh but tameez.
Tameez will be written as तमीज.
Now, if even if you want to write tamizh, it will be तमीझ.
One important thing to note is that there are certain Devanagiri script letters which Hindi speakers don't use but Marathi speakers use. Some of them are झ (used in tameezh), ण, ळ, ष, etc.
Now, if you come up with words that don't originate in the subcontinent, in some cases Devanagiri script won't be suitable as the letters in those words will simply not be in the script. In that case, new alphabets will have to be added to the script.
But for the majority of the words in subcontinent, the Devanagiri script is perfect and as I mentioned you literally write what you speak and speak what you write without ambiguity. The Roman script has many words that are ambiguous with people having to second-guess the pronunciations.
I think that at the end of the day, it comes to trade-off. The Roman script has limited alphabets so learning it for new people is easier but it comes at the cost of ambivalence. The Devangiri script has a vast number of alphabets, velantis, etc to cater to every twist and turn of our tongue but it can be difficult for some people to learn quickly.
If the Roman script would have been near perfect like the Devanagiri there would hardly have been an Indian accent to English as Indians would have literally studied all the English in the original correct pronunciation. I hope you understood what I meant here.