Not sure of the size of your rods or your target, but in general saltwater critters have a little more power than the freshwater.
My experience has been that the general water pressure plays a larger role as well. In rivers there is more soft water, where in the surfline and general tidal flow there is far stronger currents at play. A small fish turning in current can really fight harder with less effort.
In terms of salt and sand. Its just a horrible combo for a jointed rod. So easy for sand to find its way into joints. It requires care.
I am using carp rods…they are about 250…USD. I think they are around a 57-68 penny rating. I can cast a 4-6 weight flyline as my mainline. I have also fished them with furled leaders but the leaders do not load the rod very well.
I bought the carp rods to target both salmon and striped bass up to 15 #. Most of the fish I have caught have only been in the 2-4 # range and include fluke.
As a tip. I have found flatfish easier to land if landed side-armed instead of lifting. Less fish lost and they tend to glide in once you turn their head.
If I were you I would not risk your tenkara rods in the salt. I would find a budget 5m rod that will accept a minimum 4-5X tippet…something on the 40 penny stiffness rating.
the nissin 540ZX might be good…not that is a budget rod, but close to the other criteria.
Its not to say that you cannot just use a regular tenkara rod. You can probably do it…you may ruin it though. Either the salt/sand or hooking into a larger species that you did not intend on as a target.
I see my salt rods as being sort of an experiment. I take apart my rods and soak the segments after an outing. On a couple of occasions, I heard/felt the grind of sand in the joints. In the field, I did my best to rinse out the joint the best I could…then did a more thorough job at home.
It is a lot of fun for sure, but a far less practical tool compared to a fly rod. You really need to pick your spots and conditions to use it. Sort of a super niche set of conditions. I personally feel that even a fly rod has little place in the New England surfline, and the fixed line version, even less of a place. Its just not the best engineering for the task. For trout in streams…yes! For the surf…you really want to pick your moments.