Hi mrfurious214.
I am glad to hear that you are happy with your Spindle mattress. 
It looks that you are in need of a little more surface plushness of the upper comfort layers to encourage good blood flow to the joints and muscles. Generally, as we age our need for pressure relief (thicker or softer comfort layers) can often become greater.
I agree that a plusher topper or even a mattress pad added to your Spindle’s soft configuration … consisting of three 3" layers of Dunlop (see specs here) is a good way to fine tune the mattress to your new needs. Because both you and your wife are fairly light I’d keep in mind that thinner and lighter body types generally do better with thinner and softer comfort layers than heavier body types. When selecting a topper for a topper/mattress combo I’d make sure that the deep down support needed for proper alignment is still maintained. There are some topper guidelines in post #8 here that may be useful as a guide. While it is not possible for anyone else to know with certainty which topper you’d do best with because of the many variables involved and your own personal preferences, I’d probably start with calling Spindle themselves to ask for their recommendation. They used to carry Mountain Top Dunlop toppers and while I don’t see the toppers listed on the site now they may still have them available or give you an idea of what might work best for you based on the averages of consumers similar to you.
As far as instructions for selecting a topper, there is more information about choosing a topper in post #2 here and the topper guidelines it links to, which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market), can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
Because of the uncertainty involved with purchasing a topper where you can’t test the combination in person … a good exchange/return policy can also reduce the risk of an online topper purchase so I would also make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case a topper you choose also doesn’t turn out as well as you hoped for. There are links to topper suppliers and some with return policies in the link I provided in the previous paragraph.
At a quick glance, listed below in alphabetical order are some of [url= https://mattressunderground.com/the-underground/our-trusted-members.html]our trusted members[/url] who offer latex toppers that you may wish to consider. I’d keep in mind that most of the toppers being offered are 2"- 4" thickness but some of them have srhredded latex, pads and wool topper options as well that might be suitable for you.
Arizona Premium 2" or 3" Talalay & Dunlop toppers
Arizona Sleep EZ 2" or 3" NR Talalay & Dunlop toppers
Cozy Pure a wide selection of toppers & pads (shredded latex or wool which might be just what you need)
FloBeds shredded latex and wool toppers
Foam Sweet Foam 3" toppers both Talalay & Dunlop
Latex Mattress Factory 2" or 3" toppers
Luma Sleep here has 2" & 3" Talalay toppers (3 firmness options)
My Green Mattress Dunlop toppers
I hope this helps and let me know if you have more questions and I’ll do my best to assist you.
Phoenix