I had a similar problem with my fttc, i had countless engineers out who couldn't stop the line erroring.
In the 70's there was a shortage of copper so from the cabinet to the property aluminium was used this was more prone to interference and bad line attenuation.
What really worked for me was emailing Clive Selley ceo of open reach,complaining about the condition of my line,his office responded within a few days for more details and i was contacted by the Sky executive team.One person was handling all the issues with my line and these guys really go above and beyond,i had multiple resets and the cost of service got discounted.The line stayed on 80/20 fast path for over a year.
There's 2 different modem types based on chipsets from Broadcom and Lantiq,giving different results on the line.
It all comes down to if you feel your connection is acceptable or not, and if you're happy with the results.
Edit 12/11/2020 -
My friend had major issues with his line and has been on interleaved, he recently started using a asus dsl-68u with the modem built in.
Initially connection remained interleaved,after installing merlin he went onto fast path with a depth of 1, he went back and forth and everytime he used stock fw or his isp modem connection was interleaved, merlin wrt was giving fast path but at lower speeds i guess its how it maintains stability.
Theres also a program called the asus tool where you can directly change dlm parameters not recommended unless for advanced users, i believe its one of the reasons openreach removed the 68u from the approved modem list a few years back.