I'm sorry to hear that your finances prevent further testing because my first recommendation would be to see a specialist - a veterinary neurologist. Cluster seizures do often have an underlying cause that a specialist can detect. Since that isn't possible, I'll give you some other information.
The seizures will have an effect on your dog's quality of life. As you've already seen, it can take a long time to get over the seizures. They can also cause brain damage, which would have a great effect on quality of life. Cluster seizures can also progress to what is called status epilepticus. That is a long seizure that doesn't stop. There is no regaining of consciousness in between like there is with cluster seizures. Status epilepticus is a dire emergency and often results in death if it isn't quickly treated.
With all that being said, there are some additional things you can do to help your dog. I recommend that you speak to your vet about the possibility of obtaining some rectal suppositories of Valium. They have been found to stop cluster seizures, and are often given to dog owners to use at home. Being able to stop the seizures right away will greatly reduce their effects on quality of life. The following site has information on this treatment. You may want to print out the information for your vet.
It isn't possible to completely rule out vaccines as a contributing factor. You can reduce that possibility in the future by getting only required vaccines, and getting boosters no more often than every three years. Vaccines have been found to be effective for 3 to 7 years, possibly longer, in most dogs. Many local vets continue to give yearly boosters, while the veterinary teaching hospitals and the American Animal Hospital Association now recommend less frequent vaccines. Recommendations are similar for dogs, and your elderly dog probably only needs the legally-required rabies vaccine. I suggest that you read the information on the respected UC Davis site before making a decision.
Artificial colors and flavors are another potential factor. You should get your dog on a food that doesn't contain them. At the following site, you can find more complete information on choosing a dog food and all aspects of feeding a dog.
Among the better brands are Solid Gold, Innova, California Natural, Castor & Pollux, Canidae, Eagle Pack, Old Mother Hubbard, and Evanger’s. You should be able to find some of these in larger pet stores. Some vets sell them, too.
I also recommend that you go to these two sites for more information, including different medications that may work:
Toxins, such as pesticides, cleaning products, air fresheners, scented candles, etc. may also be a factor in seizures. In beagles, stress is also a precipitating factor, so an effort should be made to keep the environment as calm as possible.
Beagles are one of the breeds especially prone to seizures. The American Beagle Club has a whole page on the subject on their site. You will certainly want to read that. The site states that a drug called Mysoline is especially effective in beagles, so that is something else to ask your vet about. Here is the site:
They do state that such seizures can usually be brought under control. If you're thinking that euthanasia may be necessary, I wouldn't consider that until you look into the options of rectal Valium, and trying other medications. If you have more questions, just let me know by clicking on REPLY. I hope your dog's seizures can be stopped.