I have RS Lyrik 160 Uturn mission control but I don’t think I have every used full travel (firm spring fitted)
The sag ring is always on the R or N after riding.
I’m 14.5 stone loaded so a firm should be fine.
Thought please

maybe the oil weight is too heavy? or maybe too much of it

I’m maybe a little bit lighter than you and I run a medium in Lyriks. So maybe you need a lighter spring…

How much sag do you run at the moment? On a Lyrik I’d be looking for around 30-35%

What do you actually do with it? I rarely use all the travel in mine, because they’re set up for the hardest use they get, but they don’t do that very often. The old “use full travel every ride” thing doesn’t work if your riding varies and a Lyrik’s such a versatile fork.

Forks are mainly used for trail centres and the Quantocks with the odd uplift at FOD.
Ive rode the firm spring scince ive owned them but changed back to medium tonight.
Its a super simple job so might just use the firm for uplift days.