I would carefully review the exact wording of each of the deeds as well as the Plan, and see what clues they give as to how to divide the maintenance responsibilities. You quote their deeds as saying, "...grantees, for themselves, their heirs and assigns covenant with XXX Owners their heirs and assigns that they will share proportionately with other lots with access over said right of way in the expense of maintenance..."
I assume that XXX refers to you and the other landowner over which the access runs. If that's the case, the phrase "covenant WITH the owners" sounds to me like it could be argued you and that other owner also will share in the cost.
The question is, what was intended by the word "proportionately?" I know that a lot of people (particularly those who are closest to the public road) argue for basing each person's fee on their distance from the main road. But that can result in the people farthest out paying a whopping fee, and getting them to pay that much may be problematic. In order to really calculate it fairly, you'd have to take into account that the 8th person out should only pay 1/8th of the cost of maintenance of the first section of road, which is used by everyone. Then they should pay 1/7th of the cost of the next section of road, which 7 people use. Then they should pay 1/6th of the cost of the next piece of road, etc. Each of those sections would have to be calculated according to footage. Then you could figure in the fact that some sections of road are more expensive to maintain, due to hills, culverts, low boggy spots, etc. In short, trying to figure out a "proportionate" share can be difficult if not impossible.
Many road associations have come to the conclusion that the best option is to simply charge everyone the same amount, which assures that no one will pay an exorbitant amount anyway. (On our road, each owner pays about as much as they would to have their own driveway plowed, but what they get is year-round maintenance - plus a feeling of community. So what is there really to complain about?) Under that system, it should be agreed that in return for sharing in the cost of maintaining the whole road, that everyone in the road association has a right to use the whole road if they should desire to do so - even if it's just to go for a stroll or walk their dog.
One option we offered to the first people on our road when they complained was, if they didn't want to be contributing members of the road association, they could move their driveway so they enter from the main road, and block off their old driveway that enters from the association road. If they will not be using the association road at all, they will not owe dues. (They have paid their dues ever since, without complaint.)
What ever is decided by vote at the road association meeting should be worded so as not to conflict with the language that is in the deeds or on the Plan. If you have bylaws, they should also be carefully worded to agree with what it says in the deeds. So the question remains, what exactly is meant by the wording in the deeds? My guess is that if you asked multiple attorneys what it means, you would get multiple answers, and would pay each of them for their answer - which might or might not prevail if it were to go to court.
If it looks like there's a possibility of having it end up in Court, my strong recommendation would be to obtain a professional Mediator. Have them work with the members of the road association to find something they can all agree on, and then put it in writing and file it with the Registry of deeds. Pay the extra fee to have the Registry cross reference it to each of the properties so when lands change hands, the agreement is clear to the new owners from the outset. I highly recommend Family and Community Mediation as a cost-saving alternative. In the mean time, sign up for MARA's fall conference on Oct 7th, 2023, as one of the speakers is a Mediator Trainer who is going to help us learn some of the skills any of us can use to de-fuse disputes.