As with other plants, you should start acclimatizing your cactus to the outdoors and strong sun in early spring--meaning when we have stable spring temperatures--an oxymoron in Colorado!  Take them outside on warm days for a few hours in the middle of the day, extending the timing for at least two weeks until you can leave the pots outside all day and night.

Most prickly pear species (Opuntia sp.) will flower in late April or May so you want them in the ground a few weeks before that to overcome transplant shock and start growing new roots. If you don't have flower buds, you can plant anytime until early summer.

The next thing is to ensure good drainage--as you suggest with soil amendments.  You may want to create a small berm or hill or plant in a small raised bed against your south wall.

A noted cactus grower in the Denver area recommends a soil mix of 10% compost, 30-40% coarse sand and your own yard soil for Opuntia species.  If you have backfill or really heavy clay in your yard, increase the sand or use larger amendments such asexpanded shale, squeegee or pea gravel.  You can get these amendments from big box stores, nursery centers or sand and gravel companies for larger quantities.

Be very careful when transplanting.  It's easy to hurt yourself and the plant while trying to avoid the spines.

Here is little more on winter-hardy cactus:

https://planttalk.colostate.edu/topics/annuals-perennials/1028-winter-hardy-cacti/