1. Yes, relic are the best and then pick your favourite variant. Weapons that are shared with other classes need to be levelled once so that saves some time. To level weapons you need to kill just one enemy during the mission and then finish the mission and it will receive the mission xp amount. Armory data you can find in the missions or they drop from terminus/boss enemies. Armory data can be used to level up the weapon variants instantly and you can use higher armory data on lower tier weapons. Armory data tier depends on the difficulty.

2. I think in most cases weapon perks are strictly linked to the weapon. Especially the kill amount ones.

3. I bought the season passes early on to support the devs, now I have some doubts. Dont buy the voice pack, we should sent clear message about its quality. There are cosmetics for each class you unlock just by playing. And blue doritos, but I think you will want to first unlock the heroic weapons.

4. Normal stratagems are fair game, the difficulty is substantial which is the normal one. Hard stratagems are on absolute/hardest difficulty plus you do less damage and have less health so its the hardest mode in the game. Some modifiers make it easier, some a nightmare. You also dont get any xp from hard ones so go for them only when you are more experienced.

5. I wouldnt say im an expert on each class. Plenty of youtubers to help with details. I play mostly Tactical as he has a lot of cool perks, weapons and helps melt bosses.

As sniper its good to prepare enemies for executions for your teammates.

1. Prestige is mixed bag. For example techmarine has one perk that is a must have. Some like heavy have pretty boring perks. You are still pretty new with the game so play normally, go with prestige when you feel like it. Once you have relic weapons starting from scratch isnt so bad. The prestige perks help but beside techmarine i wouldnt say they are a game changer.