Need Advice on Twilight Cleric/Fighter Multiclass

I'm currently playing a level 4 Twilight Cleric and I'm considering multiclassing into Fighter after level 6. The main reason is story-related, but I also want to make sure I'm not accidentally making my character weaker.

My character has been struggling with the idea that he hasn't been able to save everyone, and his response has been that he needs to become stronger and more capable in direct combat. Because of that, I've been considering going Cleric 6 / Fighter X.

The thing making me hesitate is that I've heard Twilight Cleric is one of the strongest subclasses in the game, and I'm worried that taking Fighter levels could hurt me more than help me in the long run. Our DM also tends to run very long adventuring days with multiple encounters and very few long rests, so survivability and effectiveness matter quite a bit.

One important detail is that our campaign is expected to end at level 12, so I'm only evaluating the character up to that point. Right now, my tentative plan is Twilight Cleric 6 / Fighter 6.

My goal isn't to build the most broken character possible, but I do want to stay effective and not fall behind the rest of the party.

For those who have played Twilight Clerics or Cleric/Fighter multiclasses, would you stay mostly Cleric, take a small Fighter dip, or fully commit to Fighter after level 6? What are the pros and cons I'm not seeing?

Character Details

Male Hill Dwarf Twilight Cleric

STR 15
DEX 11
CON 18
INT 7
WIS 18
CHA 9

We used a fixed stat array of 17, 15, 13, 11, 9, and 7. I also received a free feat at level 1 and took Resilient (Constitution) and did ASI at 4 to my con.

Well, by level 6, you’ve really gotten all the super-strong twilight cleric powers, so subclass-wise you won’t miss out on much. The big thing you will miss out on is higher level spells and spell slots. Those can be very much worth looking at and seeing what you’ll give up.

In exchange, you’ll get some big buffs in melee from fighter. Weapon masteries, action surge, second wind, a higher hit die etc. but switching at level 6 means you also won’t get your next feat until level 10.

I would note that a 15 str and 18 wis means your melee attacks won’t be as effective as your spell attacks. And with the delayed feat, it will be a long time before you get a chance to increase that str.

Are you the main party healer? If so, losing out on spell slots and bigger spells like mass cure wounds might really hurt the party.

You could also invest a feat instead of class levels to split the difference. Maybe at level 8, you take crusher or slasher (or whatever weapon you use) to round up your str and give you more melee options, while maintaining your cleric abilities.

One thing I should mention is that our DM handles feats and ASIs based on total character level rather than class level. He likes allowing multiclassing, so characters don't lose out on feats or Ability Score Improvements by multiclassing.

We also use a house rule for healing potions that makes them much easier to use, so I'm not really the party's primary healer. Because of that, I'm looking more at overall effectiveness, durability, support, and frontline capability than I am healing output.

Quote from ShmoeksU >>

One thing I should mention is that our DM handles feats and ASIs based on total character level rather than class level. He likes allowing multiclassing, so characters don't lose out on feats or Ability Score Improvements by multiclassing.

This is a much bigger divergence from the official rules than it might appear. You should probably clarify with your DM exactly how this works, since not all classes get feats/ASIs at the same time — specifically, Fighters and Rogues get more than the others — so while this does encourage multiclassing, it also makes how multiclassing works at all somewhat confusing. When exactly you're going to get those feats/ASIs, and whether you're going to get the extra ones from the Fighter class, will have a big impact on how you should structure this build.

 i just asked him and he said "Since we move the ASI to be character level based, the level 4, 8, 12, 16, and 20 ASI will happen regardless of individual class level. a 6/6 split will get you the level 12 ASI.Fighter also gets an ASI at 6, which we will keep as a fighter thing.So, a 6/6 split will get two ASI at level 12, yes
If this turns out to be a headache, we may need to walk back the previous change going forward. Time will tell; we shall see how it goes."

Well, that definitely changes the equation. It all but penalizes you for not multiclassing. 

So, the feat part isn’t really an issue. The big choice then is higher level cleric spells vs fighter abilities. Which will really come down to a personal choice.

Though I will still throw out the idea of just taking some combat related feats instead of fighter level. Or just doing neither. A role play choice doesn’t need to be represented through game mechanics. You can make a deal with a devil and not be a warlock, for example. 
Twilight clerics already get heavy armor and martial weapons. You’ve got the str for plate armor. Use a weapon, shield and take warcaster at level 8. Throw in spirit guardians and you’re going to be a front line beast without any fighter levels.

Thank you. I'll definitely think about that.

Right now, I'm still leaning toward Fighter. Our campaign tends to have very long, drawn-out adventuring days and fights, and I often find myself running low on spell resources. I also find it a bit difficult keeping track of a large number of spells and options.

Because of that, I think I may end up going Fighter and focusing more on being a defensive frontline protector for the party while still keeping the core Twilight Cleric abilities.

Clc6/Ftr6 means you don't get 2 attacks until level 11. 

So you don't get proper fighter ability until end of campaign, and for that you sacrifice spell progression.

Go for what makes you tick though, a 1 level dip gives you the fighting style you can sink feats into etc. After that you gotta look at what you want out of it.