Interesting itinerary. After leaving school in 1978 I bought a two week 1st All Lines Rover and had a great time. I enjoyed it the following year so much I bought a 14 day one. Had great fun did quite a few sleeper services to keep going I recall the 1st compartments were just £5 back then. Unfortunately by 1980 the need to do dissertations came along, then a other priorities took over and I learnt to drive.
Retiring at the end of 2019 and obtaining an over 60s railcard I intended to get a an All Lines in summer 2020 and then C19 came along.
All Lines was back on the agenda for 2022 then unexpectedly my PC went on the blink.
I thus blew my Railrover budget on a new power PC and the All Lines has been deferred to 2023.
Your planned Itinerary has given me some ideas for what I should be looking todo. Living not too far away from you I could adapt it - I like the idea of Aberdeen to Penzance!
However, I would make some suggestions at the Cornwall end.
St Ives Line is short and gets very busy as does St Ives itself in the summer, best time to visit and appreciate St Ives is out of season.
If I were you I would consider doing the Liskeard to Looe line - Looe, in particularly West Looe does not get as busy as St Ives - is on the coast and is just as interesting, The Looe Branch is also delightfully quirky with its separate Liskeard station at 90 degrees to the mainline station. This now boasts a heritage centre in its waiting room with various items of memorabilia on display and access to the business side of the former Looe Branch ticket office. Apparently the heritage centre opened in 2021. However, the heritage centre only appears to be open when the ticket office in the main station is open. When I went on Thursday it was open. When I took a second trip in the late afternoon on Saturday it was locked as the station ticket office closes at 15:00.
The other "must do" line is to Gunnislake up the former SR mainline to the junction at Bere Alston - the signal box there has recently been splendidly restored for use internally as a scout base. Here the train reverses to head up the former Callington Branch to Gunnislake. This section is a former Colonel Stephen's light railway with steep gradients and the spectacular Calstock viaduct offering superb views across the Tamar then follows the up hill grind to Gunnislake with stops at ungated crossings.
Personally I think the Gunnislake line is probably the most interesting and scenic of the Cornish branches.