You are on the right track by learning you have some limitations as f2p :)
I will try to share my personal experience and mostly my mistakes as a new player. On my first account I made two mistakes, that costs me a lot of time in term of progression. I overlooked Arena and I made dungeon progression my priority.Then I ended in some big progression walls like those about Arena, reaching Silver, making Great hall upgrades and such. I underestimated Great Hall upgrades and that in some way reflect in my dungeon progression too.
If I am about to start this game again, I will make Arena and Clan Boss my priority (after making my initial farmer to 6*, as you already have done). I still don't like Arena and just run all my keys almost always on auto with more than decent win rate, but I try not to waste any key by don't playing it. Great Hall upgrades can make or brake your progress. Clan Boss progress is another area i wasn't focusing, mainly because I haven't got any decent or high rated CB champion and building someone that will be outplayed by someone else in near future, probably, most likely... Yeah, by the time I realized that doing noting and waiting better champions is not that good plan I already have wasted a lot of opportunities.
Now, on to your points. I have never considered farming campaign for equipment a thing. Still don't. Sure, you can focus on certain pieces, there are guides out there that recommended doing this. but I disagree. Just wait for some appropriate event, go for highest level you can do on auto, play and prey. There will be bad drop all over the place, but among them there will be useful and probably way better than those uncommon and common from campaign.
Don't use epic as food, It's tempting, we all do it, but don't. I will go further by saying don't do it even if you got duplicate, unless it is a really bad champion. Recently it was announced from Plarium that they plan to implement some new mechanic around duplicate champions. When? Not sure that anyone around can tell, maybe it will take forever as usual, but if you have good epics, have vault space and not plan to use them, keep them. Someone use duplicate as books, that is a valid choice, but i just can't force myself to use my second Skullcrown as book.
Gorgorab, Scullcrusher, Crimson Helm, Grizzled Jarl are your best bet for Arena troll team. In that turn order. Or Zelotah instead of Crimson Helm or Grizzled Jarl maybe. If you have already tried those in Arena and you have some observation how they behave together on auto, i am curious to heard that. And going for Scullcrusher as next 6* is right decision, you can't go wrong with him.
Minimum ascension to unlock skill upgrade is almost mandatory even if it cost extra efforts and resource. If you have good accessory for second slot, go for full 5* ascension, if you don't have accessory that will make significant difference for important stat or a good overall boost, don't do it unless it will come with minimum effort.
*As a side note - you already have 3 6* champions: Zelotah/Spirit, Elhain/Magic, Apothecary/Magic. That is good. I don't know where are you on progress mission line (the one that will unlock Arbiter for you) but somewhere around the middle of 4 stage you will face number of missions that sucks hard if you are not prepared for them in advance. And you can prepare only if and when you know about them. The series of missions will ask you to ascend one champion from every affinity to 6*. It will not count already fully ascended champions, you need to ascend one said affinity the exact time you land on that mission. Knowing that, you better plan accordingly, as facing those missions with bunch of already fully ascended champions and finding that you need to make another four 6* champions - one from every affinity is a massive "WTF!" moment. Trust me, if i have someone to tell me just one important thing about Raid, I would want to tell me about that in advance.
You still keep reading? Well, i will continue then. Yes, do mastery, but don't overinvest. I don't know how further in mastery tree are your 6* champions, but T6 is a game changer if you want to progress, on CB especially. Some champions benefit way more from T6 than others, some don't even need one mastery to work properly in certain scenario. If you have important champion that don't have any mastery or just minimal mastery, you need to max his mastery and you have 800 diamonds (or ruby, or whatever that red thing is) - buy that champion full mastery. It is not bad investment when you are at that stage. Further in game, when you have 3 or 4 fully maxed champions that can run Mino/15 auto you can run them along with that next guy, that need mastery. Even if you buy enough energy packs to speed the process, you will most likely will spend less diamonds and will accumulate some silver (not sure, how many, didn't bother to count it, but probably around or more than half a million. Not sure really, someone can come with more accurate numbers). Also, there is a good online resources that will help with mastery if you don't know for sure what to do. Later on you will confident enough to make your own decisions about your mastery according the fact where and how you use certain champion. Even it you end making your own decisions from very begging, it is still good idea to check others opinion from experienced players,
*Do daily quest;
*turn all available arena token into medals at higher possible rate;
*try to finish challenges when possible (some good reward there);
*tryhard on progress mission when reasonable, pay attention further ahead of what is coming and plan accordingly;
*don't do tournaments for ranking, do them for whatever reward on the right column you can get unless you have big bank account or being disappointed is not problem for you;
*events is where f2p like us can take full advantage. Arena event, training event and dungeon diver event are the best. Sometime summon event can turn good too, if luck is on your side;
*run for highest possible Arena tier where you can be. You can lose all of your defense battles and thus lose arena power, but by carefully choosing your attack battles and spend all of your arena tokens at higher possible rate will keep you not to fall to lowest tier. Arena chest is awarded once per week and most of the time will not make you happy, but higher arena tier means more medals per token earned.
*Great Hall bonuses from Arena tiers and upgrades are big. Like Chuck Norris big.:) Go for accuracy for whatever important champion affinity you have at the moment, that have important debuff, then go for another important one that you use. On important damage dealers you can go for crit. dmg. That's where you priority are in great Hall. On special cases you can focus on different upgrades, but those are generally most useful overall. When you run fourth stage of progress mission you will face another series of mission that ask you to upgrade 4 upgrades to 6 level and then upgrade one to 7 > 8 > 9 > 10 level. You will want to have those upgrades already finished or about to be finished, otherwise they will hard block your progress. Not good when next dozen missions after this are already finished and you are about to just collect their huge rewards.
*keep your wallet away from game and don't encourage bad businesses practices and you are about to beat the game :)
