Sadly, it does seem GW are wising up to the discounted retailers, as indicated by the frequent reports of these stores having the quantity of new (high demand) items they can stock limited, compared to the quantities they could order in the past.
I'd have to point you to two factors here that are hampering their production and distribution network: Covid and Brexit. The combination of those are what's hitting 3rd party retailers the hardest.
Covid in particular is a big hitter. Social Distancing requirements in their Nottingham base of operations has slowed down their ability to get product out of the door, from the sheer fact that they can't have as many people as usual working in the same space at the same time. Then having retail stores in the UK come out of Tier IV lockdown only a few weeks back meant that there was increased pressure on the factories to get ~4 months of new releases into stores for people to buy. Having been in my local store a few times I've seen how their usual delivery schedule was impacted (fewer deliveries) and gaps on the shelves from stock that hasn't been shipped. If you were GW and you had a bottleneck in your production and distribution networks, where would you prioritise your supply being distributed?
So yes, third party retailers have been hit by not getting as much stock as usual - but it's also reflected in other areas, such as GW brick an' mortar stores. Even newer releases on their webstore seem to be selling out quicker than previously, and I'm sure it isn't down to increasing popularity.
A better individual option would be to just buy what you need (which, if you are honest with yourself, may be nothing), or convert things that you want, or even buy 3rd party/non-GW materials, and while all of these feed GW in some fashion (for rules, at the bare minimum), your wallet will be aligned how you want it.
This is such a good suggestion. Gravis armour isn't exactly new to the Heavy Intercessor, so converting shouldn't be too much of a problem. Kitbash some cool takes on the updated bolt rifles - I know so many people that are actually happier to see kit bashed / converted units rather than stock miniatures, playing them as counts-as shouldn't be a problem.
Price should reflect cost to produce though often it seems like you're also paying a premium for the rules that go along with stronger units.
To cover the second part of that statement, yes there is a undoubtedly a premium in pricing for those elements where customers are not going to be buying multiple boxes. HQ choices and special characters are the epitome of this, looking at examples of Primaris Chaplains / Captains / Librarians / Apothecaries. Are people really going to want to buy another troop choice when there are already multiple existing Troop choices that are plentiful enough? Even taking that into consideration there is also no doubt that a RRP will factor in projected sales volumes (variable) versus cost to manufacture (variable) and cost to develop (fixed) and cost to support (both fixed and variable elements). Basing price purely on a net weight of plastic and complexity of sprue design just doesn't happen, and frankly nor should it.
As for "Price should reflect cost to produce" - real thought needs to go into how much you think it costs to "produce" these items, and I'm guessing the thought process here is based on raw materials and time to manufacture? There are so many additional factors in play here it wouldn't be possible for a single person to accurately create a list of the many costs and overheads that a company of GW's size has to accommodate. Everything from the costs of running brick and mortar stores across the globe (500+ if memory serves) with the staff associated with keeping the doors open, through to the development cycle of creating new models and rulesets (from first concept through to graphic design, translation to sprue design, mould making, refinement, art design, rule design, management, etc) and the various background services required to keep all of that running. As someone who has considerable experience with the cost of corporate IT, the amount of money that is spent on systems dedicated to ERP, HR, Warehouse and Distribution, Security, Web Hosting, Payment Processing and Finance - they cost a literal fortune. Then there's the staff required to keep all of this running; from the people running the warehouse to those manufacturing the sprues, thorough to the support staff who keep all the building running smoothly, HR, Security, Facilities, Artists, Designers - and the fact that the majority of these people live and work in the UK, which is one of the most expensive places to have staff. A lot of other companies would have relocated their manufacturing to China for the reduced cost in production (seriously, it's over 90% cheaper), and their IT system support and Customer Services teams to India (~80% cheaper) but GW does the majority of its work in the UK with UK people - keeping jobs in the country I live in, and I think that doesn't get said enough. People aren't cheap either, ignoring base salary and looking at elements like pension schemes, paid time off for holiday / sickness / maternity & paternity leave, insurance policies, and the various other benefits you get. Look at Covid when it hit the UK. GW made a point during the first major lockdown in the UK (where all retail stores, office, and manufacturing premises were closed) that no member of staff was made redundant, and that the company maintained employee pay at 100% by supplementing the UK furlough payment scheme. They were also one of the first to announce paying back furlough money back to the UK government after lockdown was lifted. Hell, if that's where some of my money is going then fair enough.