* Could I just use Osmocote Plus to establish a base level of nutrition and then complement that with a once-a-month synthetic fertilizer at 9-3-6? Yes, though I try to limit use of controlled release products (like Osmocote) because the release rate is almost entirely based on temperature. This means that during summer's dog days when plants are semi-dormant due to heat, I'll want to be pulling back on TDS levels (fertilizer salts) to facilitate water uptake, but the controlled release products are at maximum release rates. Soluble synthetics provide the grower with the most and most precise control, but there's certainly a place for a controlled release product, especially if used moderately.

* I added photos to the original post. Based on how the leaves currently look, would you say this is a lack of nutrition that is likely to be addressed by proper fertilizer? Mandevilla isn't a plant noted for prodigious production of foliage, so it's hard to say. How rootbound would you say it is? When most of the foliage is concentrated at the ends of branches/vines, it's usually a sign the plant is rootbound. If it is, the yellow foliage could be do to low oxygen levels or uneven moisture levels, If it's not rootbound, I'd look to nutritional issues or perhaps an insect issue (mites?) as a possibility.

* I read online that Mandevilla likes a heavier phosphorous ratio, to encourage flowering. I guess this might suggest it would respond to a rose-type fertilizer? What is your thinking about ways to promote heavy flowering? I have a great deal of difficulty believing that high phosphorous fertilizers are ever appropriate for containerized plants. The average plant uses about 16% as much P as N, and no plants I know of take up more P than N, or even close. The "bloombooster" fertilizer 10-30-10 often prescribed as appropriate for mandavilla provides 129% more phosphorous than nitrogen, so more than 8X what the average plant can/will be able to use. This could only be considered to be a toxic level of phosphorous, which can severely mess up your plants ability be creating the antagonistic deficiencies listed in my post above, as well as creating a synergistic conditions during which too much magnesium could be taken up at the expense of calcium uptake. I have grown several mandavilla vines, treated as annuals, and had very good success, abundant bloom profusion, with FP 9-3-6 and a bark-based medium with dolomite as the liming agent - the 5:1:1 mix.

I see that certain sites suggest a "fast-draining mix" of equal parts of peat moss, potting soil, and builder's sand, but to me that doesn't make sense. That mixture cannot produce a mix that is well-aerated and drains to the degree the plant would tolerate it well as it's noted for it's intolerance of wet feet.

Al