Pretty new to grinding MUT

This is my first year actually taking MUT seriously and I have no idea how people are getting the amount of coins they are while still improving their team. Is there any way that is a guaranteed coin gain rather than just getting lucky on packs or rerolls?

just never touch packs... except your daily 500 coin pack. 

I would just get your team good enough to complete and roll with them as long as you can.. use free BND cards where you can..

and NEVER buy a card day 1 its released..

1. First off. DONT BUY PACKS OR REROLL. You will always lose. 

You have to grind out solos/do all the objectives in the passes and smartly invest coins. 
The biggest thing is having a sizeable coin stack, doing sets that return the cards and selling the masters. Current costs when I did it yesterday for the newest MF 90s was about 150k for 3 of them (buying the 3 you need for a set to get back in return, putting them into the highest selling master and then immediately selling that master). 
Once you get your hand at the AH and figure out what works for you to flip cards, that will always steadily increase your coins. Youre not going to find a 100k card for 1k often, youre looking for 10k+ after taxes when you sell per card. 

2. YOU DONT NEED THE TOP END CARDS.
Get cards that fit what you want to do, that meet thresholds and go from there. 
Just because its shiny doesnt mean you need it, you can eventually get it when its cheaper. A month ago, Chase young was like 200k and now hes less than 50k last I checked. Still gives you the edge threat ability and still a top end pass rusher. 
As seen in the first one, if you can manage to get like 450k/500k whatever it is, should keep you constantly able to update your cards by getting the ones to update your team and then immediately selling the master. 
You can get this team currently for pretty cheap. Except for the MF cards since they just came out. And this isnt everyone you can get, this is just who I think is the best of the bunch at the current time in that position. You can fill in others are you need. If you want to work backwards, CH is going to have one at every position then UK, then GG, now MF. CH/UK only need 5 cards for their master, GG needs 6 cards, and MF only needs 3. But you should price out the master and how much it would cost after tax from selling the master to see if you have enough.