IMO You have too much oil (linseed) in the mix for it to dry in a reasonable time.
You may have applied it too heavily also.

Linseed coatings need to be ultra thin to get them to set-up.

Smear a coating of the same thickness as you put on the stock onto a clean glass jar surface when you are doing a stock.
Leave the stock alone to 'dry'. Check the smear on the glass jar surface to see when it's hardened.
If it isn't set up on the glass surface,,it won't be in or on the wood either.

The Spar Varnish is already a linseed based varnish with a + amt of oil in it to keep the final set finish somewhat flexible to withstand outside weather. That's where the name Spar comes from,,boating use. But it's been used pretty loosely over the years to market as some sort or extra special varnish.

Then you've added more linseed oil to it.

All that oil in the mix with what little resin in the SparVarnish orig make up is what you have made as your Varnish finish.

The turp or or paint thinner is nothing more than to mix and thin the above components. It evaporates from the mix as it sets on/in the wood. It does help with some penetration into the wood due to thinning.

Linseed does take a while to harden. It oxidizes to harden, it doesn't just air dry like we think of most paint products doing.

I'd skip the extra linseed oil. Use the Spar Varnish or any decent varnish by itself. Thin it with min spirits if you like for easier handling like sanded in finish.

Thinned with min spirits (petroleum distillates!) and you have what about what is sold as Wiping Varnishes in the DIY finishing depts of Home Depot ect.
They are around 75% min spirits, 25% varnish.
The varnish portion will be the oil (linseed, tung, or perhaps another plant oil) and an resin,,either natural or man-made.

The different ratio of oil content in the varnish makes them more or less suitible for outdoor use over indoor use.
That's were you get into the differences in Long Oil Varnishes and Short Oil Varnishes,

Linspeed Oil and Tru-Oil are nothing more than Linseed oil based varnished using synthetic resins (alkyds) instead of natural resins.
Along with that there are dryers in them, synthetic also as the old Japan Dryers were lead compounds and taken off the market.