Hey Lee77uk. Hello, good morning and a very warm
to the net's best for all things Street Triple!
I can very much empathise with problems of height! Actually, as no doubt you've found, it's not simply a question of height - it's as much about the relative length and ergonomics of inseam, legs, spine, arms and neck! I've been a short@rse all my life (at best
I might have made 5' 6"!!) and shrinkage with age is only adding to the problems so you have my fullest sympathy!
My advice and opinion - for what they're worth - are to try to find a fairly sound but tatty, used seat; use the power of google to research 'motorcycle seat alterations' and have the seat foam lowered, fitted with a comfort pad and recovered to your personal preference, i.e. to look like standard or customised.
I think the idea of a specialist suspension tuner who can set the suspension up and lower it at the same time is a good idea - you need them to obtain a setup with at least a close similarity to the stock steering and ride height geometries to retain the sweet handling characteristics of the ST. I guess the thing here is to make it very clear exactly what it is you're after - outcomes, expectations and reasons - and let them make the deicisions ....... that's what you'll be paying them for!
If you're thinkng of handlebar risers you need to check the available free length of cables, wiring and hydraulic lines, otherwise you might be looking at the added expense and hassle of, at very least, new brake lines.
And finally, from me, Triumph do clear vinyl tank protector kits that are pre-shaped to give protection - and some extra grip - to the tank. However, these are, I think, obsoleted as a kit, though you might want to check that with your local dealer. If the kits are obsolete you can buy the clear vinyl as a length or roll, e.g.https://www.amazon.co.uk/MOIKE-Protection-Transparent-Scrapers-Scraping/dp/B07H3RXVSS?ref_=fsclp_pl_dp_4
A more proprietary and focused solution for tank grip would be something like these :https://www.stompgrip.com/
Not sure about "throttle spacers", I'm afraid. Are you thinking about the Speed Triple throttle tube? There's a lot of chatter about the effects of this switch but, according to a very well known Triumph ex-racer, now tech. wizard it's not woth the hassle! If your bike is working properly - leave it, or as a yank might say, "if it ain't broke,, don't fix it!"
I'll be interested to hear how you get on with some of your proposed changes. In the meantime, stay safe and enjoy that bike and don't let the vertical challenges put you off - I never do!