Hi
I don't know well templar tanks.
How much Magicka Regen do you have ? And how much magicka ? What are your skills ?
Do you use your rune ? It can help you sustaining your magicka and it'll give you Major Ward and Major Resolve which is important as a tank. What glyphs are you using on your accessories ? Adding some magick regen glyphs can help too. What is your mundus stone ? Atronach one can help too. Are you spamming your skills ? Spam less your skills. Use the stamina taunt as much as possible (if you have magicka problem, magicka taunt won't help you lol). Shield only when you need it (you'll need to learn boss's pattern to do so).
EDIT : Spinners and Rattlecage won't help you sustaining your magicka. Spinner is more oriented towards DPS people (it's a light armor set).
Having more magicka as a tank is good, but if you have sustain issue, it won't help you that much. It really depends of your stats, if you have a good magicka pool, increasing it isn't a good choice to increase your sustainability. At this point you can take look at your magicka regen.
EDIT2 : I don't understand why you want spell damage on your tank. Unless you're playing a Tank-Heal build. You need to specify if you're a pure tank or a tank-heal templar, advices won't be the same depending on your build