So tl;dr my questions:
1. Besides synthweaving, what are the best options for JK tank guardian?
2. Would switching to Biochem from Cybertech on a Gunslinger and rolling Cybertech with a Vanguard be better? (keeping my tank companion alive)
3. What are your suggestions for republic-side crafting for each class? (Guardian tank, Sage healer, Vanguard (?) and Gunslinger dps
On my JK guardian I did synthweaving and crafted a lot of great gear for him while leveling. BUT he was my first character and I leveled him prior to group finder, server transfers, and the cartel coin shop.
These conditions are important because they influence the way most played the game back then. When your server is a ghost town and there is no automated method of finding groups, you tend to solo a lot. This means that you do not have ready access to rewards from flashpoints and planetary heroic missions - all that wonderful orange gear
. So being stuck with either quest rewards or crafted gear, crafted gear wins out...if you can maintain your skill slightly higher than your level so that you can RE and have purple gear waiting for you.
Since you said in your OP that you already have a full set of orange gear, the overwhelming majority of what synthweaving can do is useless while you level.
Personally, I do not like to shuffle crew skills to "optimize;" I pick what I pick and stick with it. The reason is because I dislike abandoning a crafting skill that has a good amount of RE schematics (that I will lose if I switch).
That being said, biochem on your main character seems to be the most optimal choice at the moment, especially if your main is a tank (reusable medpacs and stims). For leveling purposes cybertech might be a better choice since you will be able to fill two of three slots in all that orange gear. But there is nothing stopping you from having an existing cybertech feed your new character with armorings and mods.
You also might want to consider artifice for your guardian. You have a cyber that can feed him armoring and mods, artifice will provide hilts and enhancements.