Imperial Coconut Stout ideas (for oak rum barrel)

I recently got my hands on a small oak barrel that held rum for 18 years. I have a simple coconut stout recipe that I really like and I've wanted to try to convert it into a bigger stout that would work well with the rum barrel. This is what the original recipe looks like:

6.5 lbs Maris Otter
1 lb oats
0.5 lb roasted barley
0.5 lb chocolate malt
0.5 lb dark crystal

1oz willamette at 60min
Add 1lb of toasted coconut in secondary.
Ferment: 65 w/ US05 or 04 (I prefer with 04 but the 05 also works well).

Any suggestions on how to make it bigger/fuller/thicker? I'm aiming for at least 10% abv. If you have any criticism or suggestions of other recipes to put in the rum barrel please feel free to write them below! I appreciate any help at all!

 

10 gallons (barrel is 20 gallons but we would split it into two separate batches and then blend). The recipe I posted was for 5 gallons in case it wasn't obvious.

 

Yeah sorry should have included that in the details. Barrel is 80 liters so a bit over 21 gallons. Right now I'd like to get a big batch only for the barrel but I'll likely be repeating this recipe several times with other ingredients (coffee, vanilla or something).

 

I do a single infusion at 153 and mashout at 168. I've been relatively consistent, getting an OG of 1.040-042 and an FG of 1.018. My idea was to create a larger version of this beer (pumping up the OG and abv) to get a bigger/thicker beer that would complement/withstand the rum flavors well. I'm not sure how to start though, that's what I was asking for. Or if you have a barleywine idea that could work that might be interesting as well.

 

Base beer came out very well, nice vanilla/cocnut aroma and flavor. A bit faint though. A friend helped me tweak the recipe to make it bigger, roastier. Gonna add more coconut and possibly chocolate too. I'll be brewing it soon.