Since you're tinkering with things a bit, I hope that you don't mind if I play Devil's Advocate, and throw a couple of ideas out there.
As far as line material, yep, Spectra's the way to go. At the size you'll need, it's; strong, slippery (read MAJORLY UV resistant, ditto on chafe resistance), & fairly inexpensive.
While in theory, a topping lift only needs to be strong enough to support the boom (and mainsail's) weight, in my experience anyway, it also needs to be strong enough to handle someone falling onto the boom as the boat takes an odd bounce or three from a wave(s). Especially once in a while when reefing, regardless of one's reefing setup type.
For a neat trick, that'll let you retain your 2:1 purchase
, in addition to making it adjustable at both ends, you could:
Splice an eye into the Spectra above the rear end of the boom, & hang a small block there. Then run a piece of double braid from one side of the boom (where it'd be attached to a micro padeye), through said block, & to a cleat on the opposite side of the boom.
Followed by (on the Spectra portion) having said line going up to the masthead, through another small'ish block. And down almost to the deck
, where you splice in an eye for a small block at this terminal end also. Where, at the deck
, there's a small 2:1 setup akin to the one at the boom's outboard
Thus you wind
up with a topping lift which is adjustable at both ends. Something which can come in rather handy at times (and, all in all, built for not too much coin).
Or, option B, would be to run say, a piece of 1/4" Amsteel Blue, from the boom's Clew end (where a shackle's spliced on), up to the masthead, through a substantial block, & down to a 2:1 purchase at the deck, or leave it long enough to reach a winch
Then you'd have an emergency
, back up, mainsail halyard
(albeit, possibly with a bit o' tinkering required). Though, a bit of cover would need to be sleeved onto, & properly secured to, the tail end of the Amsteel (depending upon your setup). But that's an easy 5min. job.
Also, a nice rig, on the cheap
given it's features (used bronze & stainless headboard shackles are like a couple of $).
Plan C's to buy a quality, pre-loved, rigid vang. They come up for sale
quite often, in a number of places. And offer support almost akin to a gallows, when properly fitted & attached.