Yeah you hit a spot where it is difficult to grind on your own as a solo new player. Your right in your assessment that interception and defense missions are extremely difficult solo and at your level. You may want to look for groups to do it with.
As for the two levels you have not unlocked there's a work around which is to find
a player that has a mission or planet unlocked can form a group with a player that does not and allow them to participate the mission and get the awards. This is called Taxing. This may be viable option for you complete some of the special limited time lotus missions.
As you said the difficult of the game has had the effect where generally veteran players are typically friendly and will be willing to help new players out. You may find some random strangers willing to help you complete the mission in even something like recruiting chat.
"New player, LF help with Anniversary Gift of the Lotus Alerts"
To my knowledge there's a newly formed GB PS4 Clan. https://www.giantbomb.com/warframe/3030-38788/forums/the-beastcoast-shlooters-new-ps4-clan-1864389/ to see if you can get a few duders to help you grind and farm for some of those resources and that rhino blueprint.
Companions like any other gear only get better through mods. Companions have what are called Precepts that give them abilities. Taxon has the ability to convert its damage to shields. There's also a plethora of sentinel mods that will increase its survivability. More heath more shields more armor. Vaccum is a really useful mod that picks up loot around you automatically within a 10 foot radius.
Note: Companions precept can be prioritized (Increasing the likelyhood they will use their precept) based on how you have them laid out, where the top left slot of is the highest priority and the bottom right slot is the lowest.
Ex. my Taxon will prioritize activating Vaccum the most, then mocular conversion, then re target etc.
(and no I don't expect you to have these mods just an example!)
Good luck duder!