It’s a free country, you can do as you like. TB X Friesian wouldn’t be my choice for a cross, but there are some people around would argue with that, I am sure- there always are. Is it a good idea and will it work out well, with everyone happy and alive afterwards? Unknown in advance.
Risk is inherent in breeding horses. If you can not accept that, best not to breed horses. The risk of loss is the risk of loss of life, economic loss, and emotional loss. The pay off is success- joy, success in competition, economic bonus.
The choice is yours. Will the mare become pregnant? Will she carry to term? Will she survive foaling? Will the foal survive foaling? Do you know how to care for a pregnant mare and feed and train a young horse? Can you afford the vet bills should things go badly? Will you produce a quality foal in someone’s opinion other than yours? The answers to these questions are unknown to us here in internet-land. It does no good for anyone to produce a foal that ONLY YOU feel has value, because times change and you may need to sell this foal at some point in the future, even if that is not your plan at the moment. If you can’t find someone else who thinks the horse has value, then you are only breeding a meat horse (which lots of people do, apparently- I see meat horses being bred (in my opinion) all the time). Horses without value to someone for something still have value for meat. Starting with a TB mare is a good place to start, TBs and TB crosses to a variety of other breeds can be marketable for sport, if the mare has quality and all else is successfully looked after. Using an experienced mare, who has successfully produced and raised foals in the past is a good place to start. Older mares carry some risks that younger mares may not, but have other advantages that younger mares may not have.
IMO, a mare must prove herself in some way, in her owner’s eyes, to earn the right to reproduce. If this mare has done that in your opinion, then one may start to accept the risks involved in breeding her. You, as the owner, must make the best decisions you can for her in terms of a stallion, taking into account economic factors, her flaws, and the stallion’s flaws (all horses have flaws). You have researched the history and performance of the families of both horses, and hopefully met a few of the offspring of both yourself. You have a history of competitive success yourself, in your discipline of choice, so that you have some idea of exactly what you are wanting to produce, what you are looking for. This is called “experience in the equine industry”, and those without it probably should not be breeding horses.