Our Bible has the right answer for ye, if your eyeball offend ye, pluck your eyes out. If you want a less draconian answer chop the trees down. After the lack of a good humor man answer, I still leave you no where.
But still without an exact answer to what kind of tree you have, its hard for me or this forum to say much in terms of what trees will fit your situation and have a longer potential. Its why you should consult someone locally expert in forestry. Usually cities and Universities have such and they charge zero for their services.
The other comment is trees are an investment in time. Sure you can buy an easily transplanted desirable tree for cheap when its only 1.5 feet high, but it will cost 20X that cost if you buy that same tree at 4.5 feet tall, and then it may take 5 years for its root system to re-establish itself and start growing vertically again.
My mistake in buying property 15 years ago was in not chopping my existing silver maples down when they were small. And those small quality trees I got and planted 15 years ago have finally grown to respectable height. While those silver maples have grown
and grown into 70 foot tall monsters always dropping huge limbs. In another 20 years they will be at the end of there lives and we be always a mess. In short avoid fast growing trees like the plague. Its never ends well.
So in closing to the OP, you got six years invested in your trees, find out if they have long potential for saving or just bite the bullet and start over. Because when it come time to sell your property, trees add many thousands of dollars to your resale value.