Sure than you might not need the dual-xxxhost isolation modes, but the might still save your bacon when one of the other devices decides to use an significant number of concurrently active flows/streams (which might never happen in reality).
Well, for VDSL2/PTM the only method I know is to look into the relevant ITU standards (in the G. series). Once you understand what is actually packaged into those PTM-frames you can start to make educated guesses...
Well you also need to take things into account you know about the link (in the DTAG case the fact that a dual VLAN tag is used at the BNG and a single VLAN tag is used on the VDSL2-link itself).
Since I was describing this in a similar thread, let me just cite myself from (https://forum.openwrt.org/t/sqm-flow-offloading-vlan-tagging-and-gaming/25113/14?u=moeller0)
"Well, on a Telekom vdsl2-link the actual overhead on top of the pppoe-wan device is actually 34 Bytes (8 bytes for PPPoE, 22 bytes for the ethernet frame (src-mac(6), dst-mac(6), ethertype(2), frame-check-sequence(4), VLAN(4))) and 4 bytes for the PTM overhead."
"DTAG actually uses a traffic shaper at the BNG/BRAS level, [...]" that accounts for the double VLAN tag which makes up for the "missing" PTM overhead.
It also helps that for the longest time 1TR112 documented 1526 as the maximum frame size on DSL-links, but that got changed recently to 1590 or so. But it is super unlikely that this has any relevance for a Telekom branded link ATM (I expect the use of baby-jumboframes in the future or MTU 1508 towards the BNG, so that the internet-visible MTU increases to 1500, but that is idle speculation).
Since I have too little experience with this router I will withhold judgement.