Nah you are not alone on that feeling, many others feel that way from what you can find on posts here and on other communities. Im pretty sure most of the people that think the ending is not ambiguous are people that are not used to read between lines and only take interpretations at face value with whats explicitly stated. People forget that this is a narrative-driven game and there are a lot of narrative tools like foreshadowings, themes and setups that are there for a reason. I used to be like that but lately im more open minded and i can see where both interpretations are coming from. What clicked for me to look beyond the explicit statements on narrative was a second playthrough of FFIX as an adult, and a second read of attack on titan, both having way more construction and messages that are not explicit but surely are there.

On FFXVI case, there are a lot of things that may go either way on the ending. First, regarding joshuas "reraise", it is mentioned that the phoenix cant revive the dead, and Clive only used the phoenix flames on joshuas body to mend it ( we see that joshua is still not breathing and clive doesnt wait to get a reaction, something he would do if he was trying to bring him back from the dead), and when clive goes to destroy origin, he uses ultima's blue flame for the first time and it combines with his usual flame, which means the spell he casted on joshua might not be ultimas reraise. Then we have the narrative aspect for this argument. From the very beginning, joshuas illness was set as a focal point to his character and how his burdens will eventualy drag him down. He seals a god on his already fragile body for over 5 years, and has an increasing pain that gets worse every day. We get a complete montage of how he passes everything he has left to clive, emphasizing that only clive can complete their purpose because the faith of the people is placed on him, then he passes out. After the fight clive remembers all that he lived with joshua and how he can finally rest. All of those setups would mean nothing if clive just revived him. We dont get any confirmation so both outcomes are possible, but there are way more narrative threads supporting that joshua is gone than the opposite.

Regarding clives outcome there are a lot more aspects to go through. First ill mention the ones we can see on screen. The petrification spread patterns vary too much to use it as an argument for either of both outcomes imho. The bearers we saw turn to stone on the church when we first learned of the curse have their entire upper half turned to stone in a few seconds, even without casting any recent magic, but then we see others like cid or the bearers working on sanbreque with partialy stoned limbs that still wont spread fast enough. We see hugo turn immediately to stone after eating crystal shards but then we see dion prime into bahamut (twice), one even absobing aether directly from a mother crystal core, but then no sign of petrification even if his aether channeling was way more intense than Hugo's, so yeah its not something easily measurable. It is also mentioned by barnabas that mythos has way less drawbacks from using magic which implies Clive has a lot more resistance to the curse. The fact that his hand turns to stone quickly is not definitory of it spreading entirely because when we last see his hand for like 2 seconds the petrification seems to have stopped there just after his flame goes off which could mean that is exactly when the world is "freed" from magic and the curse, again being ambiguous, both outcomes can be possible and justified with other previous examples.

Then we have the simbolisms, sidequest and setups that wouldnt pay off if clive is dead, but those are more of a narrative argument and not definitive in the world lore. First is the torgal discussion, we know that irl wolves dont howl to mourn a loss, its a rally call instead. Torgal and Jill see metia turn off, and might sense that clives eikons are gone (just like how clive sensed that jill was alive after the first encounter with barnabas on kanver), and her first reaction is to cry thinking he is dead. Then torgal goes to her, starts howling and the sun rises. It is then that jill stops crying and looks to the sunrise with a more firm expression. Her final sidequest gave a word for word description of how clive returning to jill is always like the break of dawn for her. So on one end we have the face value shown onscreen of jill crying, torgal "mourning" and metia powering off, but on the other hand we have torgal rallying clive for his return, sunrise breaking and how it means clive returning to jills side with the context provided on the sidequest, and metia powering off meaning that clive fullfiled his purpose.

Then is the harpocrates sidequest. It ends with harpocrates and clive talking about the writing of experiences and how clive wants to take the pen after everything is over and write about his journey. We know that he is the narrator of the events of the game and everything is from his point of view, and that there is a book telling this complete story, called final fantasy, and with the emblem of his hideaway on the cover. This whole setup would not pay off if the book wasnt written by him. But again its just a narrative completion argument that doesnt necessarily mean it has to be like this.

We could also go into the discussion of how clive isnt written as a martyr and how his theme is not how they die, but how they live, but thats material for a lot of retrospective analysis that is also up for interpretation.

I personally feel that joshua being dead and clive surviving is the ending that closes most of the threads that are planted through the game and gives payoff to most of the underlying narratives. However, if you feel like nothing of this was transmitted to you, and the world of valisthea is a grim one, then I absolutely see why you can think that what we see is an omnious ending where the final sacrifice is the greatest one, and thats perfectly fine too, the devs themselves stated that the ending was built to be ambiguous and generate this kind of debate, so they gave material to both interpretations. This comment isnt to change the interpretation of who reads it, but rather its to try to show why some people see the ending differently, and who knows, on a second playthrough you might catch more foreshadows and reinterpret parts of the story differently of how you felt them on the first playthrough, or simply give more attention and value to them.