First Big Bike Recommendations?

Hi All, first time poster here, I’ve looked through the archives but can’t find the advice I’m after, so hopefully someone here can help (my instructors seem to favour Japanese bikes).

I started learning on a Yamaha XSR125 (UK licence), and by the time my tests come round this summer, I’ll have approx. 1,000 miles under my belt on that. I’m weaker on the slow-speed stuff, so focussed on car park practice at the moment.

I’m set on getting a lightly-used large Triumph (I originally wanted an old BSA or similar, but have decided that modern things such as disc brakes and ABS are a good idea for a newbie). My use case is weekend morning tours, A & B roads, no urban or motorway use, don’t need it for commuting or foul weather riding, got a secure garage to keep it in. I’m 6’1”, 100kg, wrong side of middle age, got some back problems and a dodgy right knee - there’s no issue with supporting the weight of the bike on it, but it hurts like hell if I’m squashed on a small bike for a long time.

I measured up seat to peg distances on the Scrambler/Bonneville T120/Speed Twin 1200, and they’re all about 520mm (Scrambler is 530) - I’d prefer slightly longer, but it looks like that’s what I’m stuck with. I prefer the looks of the Bonneville, but the high riding position of the Scrambler is attractive both from a comfort and safety point of view.

My question is, will a 1200 be too much for a first bike? Appreciate it’s not a sport-tuned engine, I want something big and lazy, but is the weight of the bike a concern as a beginner?