The AVM devices are relatively safe against bricking (bootloader based ftp recovery).
JTAG is always SOC specific, unless you're very intimately familiar with the SOC in question, it doesn't help you the slightest. Even if the JTAG footprint is left on the board, that doesn't guarantee that the supporting chicken food (resistors, capacitors, voltage regulators, level shifters, etc. pp.) is left in place. It's great if you are speaking ARM/ MIPS assembler fluently and are an avid electronics hacker, with a deep seated knowledge of reverse engineering wiring diagrams from existing PCBs, but it's not a fool proof recovery method (reflashing spi-nor externally kind of is, serial console access might be, if the bootloader cooperates; JTAG is not).
Let's be realistic here, the aforementioned dedicated business class xDSL modems are as hostile against hardware/ firmware hacking as it gets, but they're also ~15 bucks on the used markets (businesses don't buy used, home users tend to have different preferences, so these are unsexy and often cheap).
xDSL as a technology is pretty end of the line, there is little future development to expect (certainly in the fftc case, maybe-maaaybe for the fttb/ in-house usage scenarios), ftth and docsis are 'the future™'. The ipq4019/ 2x2 802.11ac based F!B 7520/ 7530 (~40 bucks used) is already at the lower end of the performance/ feature scale, but currently the best supported device with the most capable VDSL2 + super-vectoring modem available. If you want better, you need to get a dedicated xDSL modem in combination with a better 802.11ax router.
If you go the dedicated modem route, what exactly do you expect would 'break' it? OEM updates may appear once or twice a year, hopefully the vendor has tested their updates, what do you expect to break there? Obviously the situation changes if you actually want to develop on the modem DSP, but you aren't going to do it for the Broadcom devices (ZyXEL) and the (lantiq based) Draytek ones are as hostile as it gets in that regard as well, so no, you'd have to raise quite different questionsanswers before even attempting that.
EDIT: if you aim too high in your expectations, the only guarantee is that you'll never achieve anything. The world isn't perfect, so make the best out of the imperfections that are available to you. Don't let perfect be the enemy of good.
If you want a 'decent' all-in-one device, the F!B 7520/ 7530 is your guy, even if it's getting a tad long in the tooth in regards to CPU performance and wifi standards. If you value router performance more, filogic 830 (e.g. gl-mt6000/ t-56) is currently the best price/ performance ratio at the upper end - and the (dedicated) xDSL modem to go along with that matters rather little (it just has to meet the OEM promises).
EDIT2: Looking more than ~18 months into the future is rarely a smart idea when it comes to IT electronics, so better to get something cheap and good-enough now (and whatever you need, once you actually do need it) - than over-spending too much, just in case. If your region is currently limited to profile 17b (100/40 MBit/s), why bother about castles in the air - chances that your ISP might uprate you to profile 35b (250/40 MBit/s) are just as high/ low than that your area will see a ftth deployment (at least around here, the later is more realistic). …and profile 17b stuff is really cheap (almost a decade out of primary service), not that used F!B 7520/ 7530 would be really expensive. Slightly overspending on the router (CPU/ flash/ RAM, …) to give you more headroom for the future makes more sense, than overspending on the xDSL modem, 'just in case'.