He had higher priorities than how his death could be framed.
It didn’t matter to Dumbledore’s plan for stopping the Dark Lord what the wizarding world thought of him or how strong he was, and didn’t even really matter what most Order members thought of him. While it was somewhat important for people (mainly in the Order) to be willing to attempt to fight the Dark Lord’s forces, it wasn’t the most important. Dumbledore’s most important plan centered around a few individuals rather than the entire Order. The Order wasn’t even involved. He wanted Snape in place to stay at Hogwarts so he could protect it, help Harry, and tell Harry about the Dark Lord’s soul in him and how he must die - Snape was the only one he’d told about the soul piece.
“If there comes a time when Lord Voldemort stops sending that snake forth to do his bidding, but keeps it safe beside him, under magical protection, then, I think, it will be safe to tell Harry.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
He also wanted Harry, Ron and Hermione to find all the Horcruxes. He was planning on them finding all the Horcruxes, and then Harry killing the Dark Lord when he faced him.
“If I know him, he will have arranged matters so that when he does set out to meet his death, it will, truly, mean the end of Voldemort.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
Dumbledore had arranged that Snape would kill him in within a year from when his hand was cursed, but he hadn’t planned exactly when - he’d said the moment would present itself. He was already weak from the potion, so when Draco brought the Death Eaters to Hogwarts, he probably considered it the moment presenting itself, and didn’t see any reason to put off the inevitable.
“Oh, not quite yet,’ said Dumbledore, smiling. ‘I daresay the moment will present itself in due course. Given what has happened tonight,’ he indicated his withered hand, ‘we can be sure that it will happen within a year.”
- Harry Potter and the Deathly Hallows, Chapter 33 (The Prince’s Tale)
As for the manner of his death being framed in such a way to make things seem hopeless to the rest of the people fighting the Dark Lord’s forces, his main plan was to take out the Dark Lord himself and that was his highest priority to make sure it succeeded. The Order members didn’t even know about that plan or the Horcruxes, so they didn’t really matter to whether the Dark Lord would ultimately be killed. In addition, there wasn’t likely anything that would convince the Order to stop fighting them, no matter how hopeless they perceived it to be. They were fighting out of “moral convictions” and weren’t the type to give up. It wouldn’t have mattered if they thought it hopeless. It’s unclear if Dumbledore had intentionally planned it or not, but the way things ended up happening the Order members got the story directly from Harry, and were all more shocked that Snape “betrayed” him than losing confidence in Dumbledore’s or their own ability - so his death didn’t end up affecting their opinions.
“Snape killed Dumbledore,’ said Harry.
She stared at him for a moment, then swayed alarmingly; Madam Pomfrey, who seemed to have pulled herself together, ran forwards, conjuring a chair from thin air, which she pushed under McGonagall.
‘Snape,’ repeated McGonagall faintly, falling into the chair. ‘We all wondered … but he trusted … always … Snape … I can’t believe it …”
- Harry Potter and the Half-Blood Prince, Chapter 29 (The Phoenix Lament)
However, the general public wouldn’t have known what happened that well, and were also being given misinformation by the newspapers that the Dark Lord controlled. Some of the people who weren’t in the Order might have gotten discouraged, but in the bigger picture, that really wouldn’t matter if they were less inclined to fight. Trying to die in a way where the general public would still believe in his strength, and try to prevent against them being told rumors, wasn’t as important as making sure things were properly in place for the Dark Lord to be properly killed.
“Which leads us neatly to the many rumours still circulating about Dumbledore’s final hours. Does Skeeter believe that Potter was there when Dumbledore died?
‘Well, I don’t want to say too much – it’s all in the book – but eye witnesses inside Hogwarts Castle saw Potter running away from the scene moments after Dumbledore fell, jumped or was pushed. Potter later gave evidence against Severus Snape, a man against whom he has a notorious grudge. Is everything as it seems? That is for the wizarding community to decide – once they’ve read my book.”
- Harry Potter and the Deathly Hallows, Chapter 2 (In Memoriam)
Dumbledore wouldn’t want to risk his greater plans to try to make sure he dies in a way that would be more encouraging to the general wizarding public, since his plan was important to ensure the Dark Lord’s downfall, and the Dark Lord’s allies would find a way to make it suit their side no matter how he’d died (and he knew he must).
He also may have been too weak to do much different.
Dumbledore was quite weak after having drank the potion in the Horcrux bowl. Harry had to Apparate him back, and he was so weak at that point, he’d collapsed on the ground.
“I’ve been better,’ said Dumbledore weakly, though the corners of his mouth twitched. ‘That potion … was no health drink …’
And to Harry’s horror, Dumbledore sank on to the ground.”
- Harry Potter and the Half-Blood Prince, Chapter 27 (The Lightning-Struck Tower)
He did actively choose not to fight that being when he’d be killed, since he’d immobilized Harry so he couldn’t help fight, but that may have also been in part because Dumbledore knew he was in no shape to fight himself out of the situation even when Harry’s help. He was in bad shape, and he knew dying at some point soon was inevitable, so he probably didn’t consider it worth the effort to try fighting, possibly fail, and possibly die in a different way than he’d intended and ruining his plan.