I use the Zyxel XMG3927-B50A, which has access to stats via UI and CLI (so it works with DSLstats). Broadcom chipset, very stable, decent features, very good wireless. I know it's not just a modem but rather a modem-router combo, but it may be worth considering. Though, expect to pay upwards of £150 for it brand new, that's even if you can find it in stock. AAISP may still stock them, so you could contact them.
Looks like there is a single listing on
eBay for one, opened but unused if you're willing to give it a punt for a smidge under £100 buy it now - says it's XMG3927-B50A-EU01V1F, but I don't know which plug type it will include (but the PSU has exchangeable plugs, at least with my model, so you could always contact Zyxel I suppose).
As for tweaking, I have never tried. G.fast targets 3 dB by default anyway. Even if tweaking is possible, it would probably be a bad idea.