I think you’ve got a couple of options here @RobH but don’t overthink it too much as you’ve got loads of time!
As others have suggested, one option is use Plan Builder and see what it spits out for you. You’ve said you’ve never trained properly so I’d definitely start with Low Volume and see how you get on. I think using Low Volume on Mon/Wed/Fri with additional longer rides outdoors or Zwifting at the weekend is a great way to do things as Mid Volume can be tough. I used the Mid Volume full triathlon plan from Base to Specialty for Celtman last year and almost cracked a bit towards the end as there’s just a lot of volume and intensity. And without humble bragging or anything I was in good shape and training really consistently. On the podcast they’re always reminding people that fully completing a Low Volume plan is better than intermittently doing a Mid Volume plan.
Another strategy (and my recommendation based on the information you’ve shared) would be starting with the classic method of working your way through Sweet Spot Base 1 and 2 then going to Plan Builder as you have absolutely loads of time before FW. I hope you don’t mind me saying but you’re obviously a bigger lad and if you know yourself you have weight that could be lost it’ll help a lot on FW as there’s so much climbing. I’ve done the route starting from Keswick and it’s absolutely fantastic but the climbs will grind you down, coming over my final hill from Ullswater through Dockray to the A66 I was f’ing burst and it’s not even a big hill. Work on getting used to structured training, you’ll lose some weight and get an FTP boost. You’re at 2.8W/KG just now, with some good training and losing some weight if you got 3.5 W/KG from weighing 100kg and an FTP of 350 you’re a different beast altogether.