Wouldn’t taking a soil pH test be the first thing to do before you destroy a tree?
Having your soil tested to determine the pH level is a good idea. Your cedars like a pH level of 5,6,7 ( 7 is considered neutral with a lower number being acidic and therefore a higher number being more alkaline) Forsythia and hosta grow best in pH 7. If the soil is too acidic for your plants certain nutrients are not available and this could be causing the sickly appearance of some of your plants.
Once you have the results of the tests you may have decide which plants need to be replaced with plants more adaptable to the pH levels. You could also try to change the pH levels to suit the existing plants but this would be required on a regular basis ( every 2 to 3 years) as the soil will naturally convert back to its natural levels.
If you do decide to remove the cedar, contacting a qualified arborist is the way to go. The City of Toronto has by-laws in place on what type of trees can be removed and you may want to contact them before doing anything. Attached is the link to their site.