I'm so glad you are feeling better about it all, and that Charlie is fine healthwise
I'm sure the Cystaid and Zylkene will help matters. You are probably wise to leave the coronavirus test for a little while in the circumstances, give him a chance to get a bit more laid back! His titers werent all that high as you say, and hopefully next time you test there will be improvement - my Gwynnie's titers were 2,500 when she was poorly, I havent retested since, as I have no concerns about her health now the way you have with Charlie.
I was just looking at the thread about your catproofing again, and noticed you were having the slabs taken up and grass put down, I would think that will be nicer for Charlie. You could also make some more planting areas so you can plant up with shrubs, grasses and my favourite - bamboos, etc - all of which will create more interest for Charlie - if you want any tips for planting I've got loads of ideas, feel free to ask. The grass that my cats like, because they can get right amongst it, is Phalaris (also called Gardener's Garters - dunno why!) its really easy to grow, and you only need one small piece to get many, many plants. It can get a bit untidy, so late summer, it can just be hacked right down, only to grow back neater and more compact for the rest of the season.
Its the one in the bottom left hand corner of this pic. I have it in loads of differnt parts of the garden, and every plant has come from just one tiny offcut initially!:
Plus you've probably seen this one of the cat post before, but its another idea (it was just a fence post in the ground using a metpost and plywood shelves fixed with ordinary shelf brackets.