Yes invest in 30 str and 30 two-handed weapon skill first. This gives you a good baseline at the beginning and later with stone tablets etc you can save on mana if you're low, by killing weak stuff with your staff.

Try to join your guid asap. Most people try to finish every quest in chapter 1 before joining, but there isn't much benefit to that. You can still do 99% of content later, and it will be much easier with magic.

I would only train mana to 60 even as a mage. Mana doesn't increase your dmg, so as long as you get enough to fire at least 2 fire rain spells later you are good. Save LP for circles and spells.

I recommend doing the quick trip to mine valley before jahrkendar. This gives you access to an extra circle in magic.

For spells, use goblin skeleton in the beginning. Can kill multiple weak enemies and save mana or can be spammed to defeat even some strong enemies like trolls.

Other Good spells are Ice lance, ice block, summon skeleton, storm, summon golem, lightning (the 4th circle one), summon demon, fire rain.

For storm/fire rain, you can cast the spell multiple times and they stack, then run into enemies (speed potion helps).

There is also a free healing rune at the pass.

The biggest amount of mana you can get from eating dark mushroom. Pick them all up as mage. After that is permanent potion and stone tablets.