Hi,
my favorite head for a full size scope is a 500AH.
Long rail to properly balance the scope (and even longer ones available), counterbalance (a mechanism which will keep a scope or camera which was balanced while horizontal from moving on its own when it is set up non-horizontally - without tightening the vertical friction - which you don't want for smooth tilts), very sturdy, does not break the bank and is about the same weight as the 128RC.
My example was bought for cheap and heavily used from the classifieds in here from a pro who had dragged it around the world and came with a good measure of dust from an unknown desert. I have continued to drag it around the world, but not quite so frequently.
About the only negative thing to say is that the plates are not Arca Swiss compatible - but different sizes are easily available from Manfrotto and 3rd party sellers.
For legs, I would look for used big brand carbon legs - preferably rated for at least double of your intended load, with as few leg sections as you can stand (more leg section = smaller diameter and thus much decreased rigidity of the lowest section and locks = less rigidity - 3 sections are a good compromise - 2 sections are not frequent - the few ones out there are often targeted at birders and recommended), and a height which is sufficient for you without extending the center column (or ideally no center column at all).
A wider spread angle is preferable for better rigidity and of course to lessen the danger of tilt.
Twist or flip locks both work well when in good working order - I personally think that with twist locks there is no lever to break and even when they start to fail you can usually give them a bit more twist until you get home...
As for brands, Gitzo is of course very good and also usually quite expensive even used - if you find a fitting example in good order for $200 or $300, grab it. It will be good.
Manfrotto/Bogen are also well known and tend to go not super cheap used.
Velbon Geo series are nice and can often be found fairly cheap - a 535 or 635 set for $200 or less will serve you nicely - I got a predecessor of the N535 for 90€ iirc... can't complain...
Then there is Feisol, Induro, Sirui et al - some models are good, some not so... read your reviews.
https://thecentercolumn.com/ has a lot of good reviews with a focus on rigidity testing.
A last word, a scopac or mulepack is warmly recommended as it really helps to carry the tripod, scope and some stuff like a guide, munchies and water comfortably on longer walks and also serves to add weight and dampen vibrations like the backpack trick.
PS: here's an image for the backpack trick
https://www.birdforum.net/showpost.php?p=2438886&postcount=12
Joachim