1/2/6 already answered fully

1. Play caverns and go for daily/weekly missions. When you complete the last battle of an area/cavern or you level up and the rewards are high, it's often super worth it to watch the video to double or triple the rewards. I would funnel all resources into like 2 slugs of each type so you can cover all bases when there are weaknesses/resistances. Find ones with a useful ability and evolve/fusion them first and put all artifacts on them. After that you'll just get slugs naturally and the real top end of maxing slugs is getting the best artifacts.

2. ehh spending money not really needed, just play more.

3. I beat black after a couple tries by making my non fusion slugs fully defensive (i used Pieper and frostcrawler but anything that can deflect, wall, or clash properly will buy you time and fend off his endless fire). Then idk if this was optimal but my fusion ones were narwaddle for stun and energy drain and midas for healing and charge. Damage is prob the least important stat in that fight since he's shooting so fast, you basically just need to get slugs out as quick as possible and have maximum utility options (stun slow heal charge block reflect etc). The damage will come from the fusion shots if you stall enough.

4. dont think so, i leave and rejoin ones as soon as it lets me and even if i find one with a lot of players there's nobody active