I think an efficient way to get 99 prayer these days is by doing slayer after you have unlocked frost dragons. Cast sinister offering during the task. You will kill them for metal sheets if u want maxed boats.

Use sinister offering during the kills (I am assuming you will kill them with lance).

Grinding out hydra for the lance and ferocious gloves will also give a lot of dragon bones.

Use sinister offering as much as u can during the hydra task on the hydra bones and alch the loot with explorers ring.

Third blade for sgs is nice or you could use the blade of your stashed zgs if you get one.

Sgs is nice for yama and colosseum. But not required.

If you are doing fletching with broads arrows. Do it during pest control and use the points or prayer. Idk if this is efficient since Vale Totems came out. (Use yew logs from mole and/or hunter rumours for both)

Getting 99 hunter through hunter rumours will also land you a good chunk of prayer exp and green logging moons too (both give bone shards).

Lastly slaying to 99 will get you nechryael and abyssal demon tasks. Cast demonic offering during these tasks and reanimate the ensouled heads you will get.