I solo a rented tandem but tried a true solo the SR Tranquility on year just to see how I'd like it.
It was a early model that had an aluminum frame work on the floor that the sliding seat mounted on. I liked the sliding seat, made it easy to trim the boat.
The downside was the frame work made putting my standard packs a real effort. One I had to lay on its side behind me and the other lay on the framework. This put both packs higher than I like, changed the center of gravity and the one behind me on its side acted like a sail when it caught the wind.
On the plus side the Tranquility rides well and handles big water better than you might think. I did end up taking my only dumping ever in it, a combination of it's faults and my over reaching.
I told Quetico Dave about it and he said he was going to lower the seat a couple of inches and see how that worked out? Next year I took it for test paddle ( empty), and I was a different animal. Still didn't have room for packs, but with the frame, there wasn't much you could do.
The NEW SR Tranquility has a completely different seat mount. Seat still slides but is mounted from the side. It's also adjustable for height.
I can't say if your packs will fit, but before loading any packs, I'd lower the seat as far as it will go. Then load and if the seat slides, figure that into you loading sequence for trimming.
Worse case scenario, you take all the other small packs and they take your big ones. I'd also consider getting a big dry bag, Coughlin makes an inexpensive one and you can fill it with water and use for ballast front/rear as needed. Emptying on portages of course

Don't lash it in, if you dump you don't want to struggle with it.
As other solo paddlers know from experience, being out in big water is no time to find out your not trimmed properly. Bow heavy upwind/ stern heavy downwind. If the rental has the sliding seat, you should be able to use it to fine tune your trim as the conditions warrant.
Good luck and wear your PFD