Hi Sonia,
You do have one approach that could work and I think bitters is a good idea for many people! However, I would consider the possibility that digestive weakness is the root cause. Yin can be deficient due to the natural aging process but it could also be due to digestive weakness. The first task is to identify the primary etiology.
Think of a healthy digestive process as a beautiful campfire. If you want to make the fire burn brightly, give it just the right amount of wood and air. If you put too much wood, it will be smothered. If there is not enough wood, the fire will be small. Since digestive fire is the function that converts food into nutrients (ie blood and then yin) the most important thing to do is to kindle the fire. Adding more wood (yin tonics) into a weak fire, the fire may smother. Bitters and carminatives can help that, true. However, the question becomes: Will bitter or carminative herbs be strong enough of an intervention for that particular person?
Assuming there is not another disease process, a weak spleen qi/digestive fire is often due to poor lifestyle. One possibility may not be to add more (tonic) herbs into the system, instead it is therapeutic to correct unhealthy eating habits and get the person moving (exercise). Have the person cut the calorie count down at each meal (unless they are underweight) so that they have a tiny amount of hunger throughout the day. A useful trick from both TCM and Ayurveda is to have the person eat a bowl of rice porridge (congee) cooked with digestive spices in place of one meal each day for a number of days. This gives the body a rest and enhances its ability to process things that are under metabolized (including stored nutrients and toxins). Also, meal timing is very important! An erratic eating schedule confuses the body's clock. The GI tract does not know when to secrete gastric hormones under the influence of an erratic eating schedule - digestion will not be optimal. Oh, and don't forget the power of the simple, elegant, post-meal cup of ginger tea!
Regardless of method, when it is time for breakfast, lunch, or dinner, the person should have a good appetite. This tells us that the body is ready to receive food. With the appropriate approach for that individual, weak spleen qi can be corrected in a month. At that point the body will be more accepting of yin tonics.
***Signs of good spleen qi: 1-3 formed and easy bowel movements daily, no distension or bloating, healthy appetite at each meal time and satiety after eating a healthy meal (no desire for snacking after).***
When you are ready to give yin tonics, continue to use a digestive enhancing herb or two in the formula. This helps to balance the cloying nature of the greasy yin nourishing herbs. Also, monitor for signs that digestive capacity is weakening. If spleen qi starts to decline during a yin nourishing protocol, it is time to back off of the tonics.
All my best,
Andrew