The Cardiff prop already has 10 Scottish caps to his name, having been eligible through his Edinburgh-born father.
But, if he doesn’t add to that tally before the summer of next year, he could then launch a second Test career with Wales.
That’s through the World Rugby current policy which allows players to represent another country they are qualified for, if they have not been capped for three years - and Sebastian’s last appearance for Scotland came in July 2024.
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The Carmarthen-raised prop won age-grade honours for Wales at U16s and U18s and could yet wear the three feathers at senior level if he continues his fine form with Cardiff, who play hosts to champions Leinster in the BKT United Rugby Championship on Friday night.
“My ambition is still to play international rugby,” says the 31-year-old tighthead.
“It doesn’t matter if it’s for Wales or Scotland. Both places mean a lot to me.
“If you ask any pro rugby player, international rugby is the next step up.
“I haven’t been in the Scotland environment in a while now. If they didn’t come in again and Wales did come calling, I think I would jump on it 100 per cent.
“In my heart, I am Welsh. I have always said it. There is part of me that is Scottish, but for me, growing up in Wales, playing all my age-grade for Wales, I would class myself as Welsh.”
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Sebastian had thought he might be in line for a Scotland recall for the current Six Nations, given how he has been going since joining Cardiff from Edinburgh last summer, figuring in every game bar one and scoring four tries.
But it wasn’t to be, as he was omitted from Gregor Townsend’s squad, prompting him to check out just where he stood.
“I thought I was playing some of my best rugby,” he explained.
“I have been playing well and I’ve scored a few tries. I thought I might be in with a chance of getting a call.
“But I never got that call, so I took it upon myself to phone Gregor and I asked ‘What’s the crack? Am I anywhere near?’
“He just basically told me they were keeping an eye on my game and we think you are going really well, but this is what we have gone with for this campaign and I was like ‘Ok’. That was it, pretty much.”
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Of course, the big question is what would Sebastian do if both countries came calling at the same time next year?
“That’s where you’ve got a headache!” he admits.
“It would be a hard decision and I am not sure what I would do.
“It would be a tough, tough choice because Scotland gave me the pathway to international rugby.
“But then, as I say, I do consider myself Welsh.”
Sebastian is actually qualified for three countries as he was born in England - in Bury St Edmunds, in Suffolk, to be precise. But he then moved to Carmarthen, in west Wales, at the age of two.
He played for both of the town’s two clubs - Quins and Athletic - during his junior rugby days and is planning to wear a sock of either team for this weekend’s Origin Round clash with Leinster at the Arms Park.
After progressing through the Scarlets Academy and earning his Wales age-grade caps, he made his regional debut in the 2014-15 season, aged 20, but was then released at the end of that campaign.
That, in turn, led to his first spell north of the border.
“Glasgow came knocking because Gregor Townsend was in charge there at the time and he knew I was Scottish qualified,” he says.
So Sebastian signed a one-year dual contract which involved training with Glasgow and playing for club side, Ayr.
“I loved it up there,” he recalls.
After that he returned to Wales and, having no professional contract, he played semi-pro rugby for Carmarthen Quins for a couple of years while working on the meat packaging line for a catering company.
His performances for the ‘Quins earned him a deal back with the Scarlets and he was to make 50-plus appearances for the region over the next four or five years.
Then, in the summer of 2021, a figure from the past came back into his life.
“I answered my phone and it was ‘Hi, this is Gregor’,” he reveals.
“I said ‘Who?’
“He goes ‘It’s Gregor Townsend, Scotland head coach’.
“I was like ‘Oh, ok, hello’.
“He said ‘We have named you in the squad, how do you feel about that?’
“I was like ‘Yeah, great, I can’t wait to get stuck in’.
“I guess I must have done something right during my time at Glasgow or he wouldn't have remembered me."
Sebastian went on to make his Scotland debut against Japan in November 2021 and shared in a 2-1 series win away to Argentina the following summer, his “most memorable” moment in the jersey.
He joined Edinburgh in 2023 and there was a Six Nations debut against Ireland in Dublin in March 2024, with his last cap coming against Uruguay in Montevideo that summer.
But, then, he went through a tough time as he suffered knee ligament and shoulder injuries, wrecking last season.
At the end of that campaign, there was no new deal on the table from Edinburgh, so he headed for pastures new.
“I was out of contract and there wasn’t an option to stay there,” he explains.
“Cardiff approached me and said they would love to have me back in Wales. I said yeah, straight away.
“I have loved every minute of it here. It’s literally a home from home for me. Things are looking good.
“I came here just to enjoy myself and enjoy being back close to family and it’s working out well for me. I can’t complain.
“The coaches are great here and I’m enjoying the company with the boys, I’m enjoying the rugby.
“I’ve played every game apart from one, so I must be doing something right!”
In terms of what he sees as his strengths, the 5ft 10ins, 18st 8lbs prop says: “I am pretty good close to the line.
“I’m pretty hard to stop two or three metres out. I don’t think there are many people who are as short as me and as strong as me!
“So, I would say close-line attack, my scrummaging and my lineout work as well. I would say they are my strongest points.
“I like to have the ball in my hand quite a bit and be like a link player.”
Looking back on his career, he concludes: “It’s been a journey. It’s a bit of a mix of everything.”
And there could yet be more milestones to come on that journey.
