So a good idea to find someone willing to let you try a single-wheel trailer with the dog before buying.
Looking at your existing trailer, if you decide to keep it, it may be worth fitting better tyres (cheaper than getting a new trailer).
A few years ago I replaced the original (moped) tyres that were on my heavy trailer when I bought it second hand in the early 1980's, as the rubber had started to perish.
I chose 60mm width Schwalbe Big Apple tyres. These are significantly fatter but much lighter than the originals, and roll much better on the flat, although they don't make much difference whan climbing hills. Another bonus is that the fat low pressure tyres provide some rudiment of suspension, useful when carrying something fragile.
