Unless there's no upgrades released that makes high prayer a necessity, it will decline steadily. As soon as you start seeing fresh prayer content which demands a greater prayer level (70-99), it'll boost again. Then again, if there aren't any new update, it'll just steadily decrease. Not a lot every week, but it is going to steadily return a little per month as years go by. It is just a volatile ability, it is somewhat like herblore.
Well, with frost dragon bots having themselves a ball, and blue/green dragons flooded with robots, I can see the price going down RAPIDLY. You may say prayer is a cash sink... directly, yes, it is. So prayer does make you cash indirectly.
Runecrafting: Probably will require the longest time, essentially, you'll want to ZMI if you've got access to lunar spellbook. Otherwise use the Abyss. Do penguins and utilize penguin points onto this. In addition, do not forget tears of Guthix because this xp will certainly be quick and useful. An alternative option would be to use the Abyss that's self explanatory or create ground runes using the FoG gloves and Digsite pendants.
Farming is straightforward and may be trained since you do other skills. Pay all farmers and repeat the planting at least once daily for fruit and possibly twice to the conventional trees as they grow much quicker. Simultaneously you are going to want to use herb and/or allotment patches. Remember xp over profit so, as an example, cadantine p toadflax.
Thieving could be trained through volcano the theiving guild. Both methods are fast and the xp you need will not take long no matter. Since you're nearly there for mining, then you likely know what to do. Granite is good xp but dull while iron could be decent xp and some money on the side. Or you can do gem rocks which can give relatively good xp with profit and crafting xp.