Just to bump and give an update on my experiences so far. I've noticed most of the HGA slabs on ebay are selling quite well and there seems to be positive feedback on some of the Youtube videos I've seen of people getting their slabs back. However, I haven't seen any cards older than 2018 in slabs yet...
Sub #1 (Fri, 2/5): 7 cards, 60 day turnaround, $20 per card. Pretty much a test run. Sent in some vintage football PC stuff (stuff that would grade 6-7 probably on PSA), a decent looking low-end Brady rookie (Skybox Dominion) and a couple new cards. HGA opened for submissions right at 9 PM EST and sold out of their 1400 card slots in about 15-20 minutes. I got an e-mail today that they received it, so I imagine I'll be getting these back in early April (or hopefully earlier).
Sub #2 (Fri, 2/12): 5 cards, 10 day turnaround, $55 per card. 1500 card slots opened at 9 PM EST on the dot and sold out in about two minutes! Since the slabs seem to be gaining a lot of traction and interest, I decided to pony up for a five-card, 10 day submission. I had planned to send these into PSA but figured I'd roll the dice since I need to cover all my card expenses for the last two months. I got a 1986 Fleer Olajuwon that I'm optimistic about (not enough to express to PSA). Also sent in a 1995 Topps Garnett RC and am going to really test them with a 1982 Scanlens Cricket sticker that not even PSA has catalogued yet (HGA actually has Scanlens in their dropdown menu, but mispelled it Scanelens - something I'll ask them to correct with a note).
Will keep you all posted!